feat(status): use SFM event forwards as primary seismograph last-seen, heartbeat as backup

emit_status_snapshot() now consults SFM /db/units (cached 15s) before
falling back to Emitter.last_seen for each seismograph. The fresher of
the two wins and the choice is recorded in a new per-unit
last_seen_source field ("sfm" | "heartbeat" | "none"). sfm_reachable is
exposed alongside so the UI can show degraded state.

Fallback is transparent: if SFM is unreachable or has no record for a
serial, the watcher heartbeat path takes over and the unit just shows
the HB badge instead of SFM. No schema changes; SLMs are untouched
(they don't go through SFM); modems inherit source from their pair.

active_table.html grows a small "SFM" / "HB" badge next to the age
column so operators can see at a glance which path is currently
driving each unit's status.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/services/snapshot.py

@@ -1,9 +1,77 @@
 from datetime import datetime, timezone
+import logging
+import os
+import threading
+import time
+from typing import Optional
+
+import httpx
 from sqlalchemy.orm import Session
 
 from backend.database import get_db_session
 from backend.models import Emitter, RosterUnit, IgnoredUnit
 
+log = logging.getLogger(__name__)
+
+SFM_BASE_URL = os.getenv("SFM_BASE_URL", "http://localhost:8200")
+
+# Tiny module-level cache: /api/status-snapshot is polled every 10s by the
+# dashboard, and we don't want to hammer SFM with one /db/units roundtrip per
+# call.  15s TTL keeps the cache mostly hot, with occasional refreshes.
+_SFM_CACHE_TTL_SECONDS = 15.0
+_sfm_cache_lock = threading.Lock()
+_sfm_cache: dict = {"fetched_at": 0.0, "data": None, "reachable": False}
+
+
+def _parse_sfm_timestamp(ts_str: Optional[str]) -> Optional[datetime]:
+    """SFM /db/units returns naive ISO timestamps (no tz suffix).  Treat them
+    as UTC, mirroring how the watcher heartbeat stores Emitter.last_seen."""
+    if not ts_str:
+        return None
+    try:
+        ts = datetime.fromisoformat(ts_str.replace("Z", "+00:00"))
+    except ValueError:
+        return None
+    if ts.tzinfo is None:
+        ts = ts.replace(tzinfo=timezone.utc)
+    return ts
+
+
+def fetch_sfm_unit_last_seen() -> tuple[dict[str, datetime], bool]:
+    """Return ({serial: last_seen_utc}, sfm_reachable).
+
+    Cached for _SFM_CACHE_TTL_SECONDS.  On any HTTP error returns ({}, False)
+    so callers transparently fall back to the watcher-heartbeat path.
+    """
+    now = time.monotonic()
+    with _sfm_cache_lock:
+        if _sfm_cache["data"] is not None and (now - _sfm_cache["fetched_at"]) < _SFM_CACHE_TTL_SECONDS:
+            return _sfm_cache["data"], _sfm_cache["reachable"]
+
+    data: dict[str, datetime] = {}
+    reachable = False
+    try:
+        with httpx.Client(timeout=4.0) as client:
+            resp = client.get(f"{SFM_BASE_URL}/db/units")
+            resp.raise_for_status()
+            payload = resp.json() or []
+            for row in payload:
+                serial = row.get("serial")
+                ts = _parse_sfm_timestamp(row.get("last_seen"))
+                if serial and ts is not None:
+                    data[serial] = ts
+            reachable = True
+    except httpx.HTTPError as e:
+        log.warning("SFM /db/units unreachable for status snapshot: %s", e)
+    except Exception as e:  # noqa: BLE001 — defensive against malformed payload
+        log.warning("SFM /db/units parse error: %s", e)
+
+    with _sfm_cache_lock:
+        _sfm_cache["fetched_at"] = now
+        _sfm_cache["data"] = data
+        _sfm_cache["reachable"] = reachable
+    return data, reachable
+
 
 def ensure_utc(dt):
     if dt is None:
@@ -69,6 +137,11 @@ def emit_status_snapshot():
         emitters = {e.id: e for e in db.query(Emitter).all()}
         ignored = {i.id for i in db.query(IgnoredUnit).all()}
 
+        # SFM event-forwards are now the primary "last seen" signal for
+        # seismographs.  Watcher heartbeats stay as a backup — if SFM is down
+        # or hasn't seen a serial, we fall back to Emitter.last_seen.
+        sfm_last_seen_map, sfm_reachable = fetch_sfm_unit_last_seen()
+
         units = {}
 
         # --- Merge roster entries first ---
@@ -93,24 +166,49 @@ def emit_status_snapshot():
                 last_seen = None
                 fname = ""
             else:
-                if e:
+                device_type = r.device_type or "seismograph"
-                    last_seen = ensure_utc(e.last_seen)
+                emitter_last_seen = ensure_utc(e.last_seen) if e else None
-                    # RECALCULATE status based on current time, not stored value
+                fname = e.last_file if e else ""
+
+                # SFM-primary, heartbeat-backup logic — only for seismographs.
+                # (SLMs / modems aren't forwarded into SFM's events store.)
+                sfm_last_seen = sfm_last_seen_map.get(unit_id) if device_type == "seismograph" else None
+
+                if sfm_last_seen and emitter_last_seen:
+                    # Both sources reported — use whichever is more recent.
+                    if sfm_last_seen >= emitter_last_seen:
+                        last_seen = sfm_last_seen
+                        last_seen_source = "sfm"
+                    else:
+                        last_seen = emitter_last_seen
+                        last_seen_source = "heartbeat"
+                elif sfm_last_seen:
+                    last_seen = sfm_last_seen
+                    last_seen_source = "sfm"
+                elif emitter_last_seen:
+                    last_seen = emitter_last_seen
+                    # If SFM was reachable but doesn't have this serial, it
+                    # means the unit is calling home to the watcher but not
+                    # being forwarded — still a working state for now.
+                    last_seen_source = "heartbeat"
+                else:
+                    last_seen = None
+                    last_seen_source = "none"
+
+                if last_seen is not None:
                     status = calculate_status(last_seen, status_ok_threshold, status_pending_threshold)
                     age = format_age(last_seen)
-                    fname = e.last_file
                 else:
-                    # Rostered but no emitter data
                     status = "Missing"
-                    last_seen = None
                     age = "N/A"
-                    fname = ""
 
                 units[unit_id] = {
                     "id": unit_id,
                     "status": status,
                     "age": age,
                     "last": last_seen.isoformat() if last_seen else None,
+                    "last_seen_source": last_seen_source,
+                    "sfm_reachable": sfm_reachable,
                     "fname": fname,
                 "deployed": r.deployed,
                 "note": r.note or "",
@@ -136,14 +234,23 @@ def emit_status_snapshot():
         # --- Add unexpected emitter-only units ---
         for unit_id, e in emitters.items():
             if unit_id not in roster:
-                last_seen = ensure_utc(e.last_seen)
+                emitter_last_seen = ensure_utc(e.last_seen)
+                sfm_last_seen = sfm_last_seen_map.get(unit_id)
+                if sfm_last_seen and (not emitter_last_seen or sfm_last_seen >= emitter_last_seen):
+                    last_seen = sfm_last_seen
+                    last_seen_source = "sfm"
+                else:
+                    last_seen = emitter_last_seen
+                    last_seen_source = "heartbeat"
                 # RECALCULATE status for unknown units too
                 status = calculate_status(last_seen, status_ok_threshold, status_pending_threshold)
                 units[unit_id] = {
                     "id": unit_id,
                     "status": status,
                     "age": format_age(last_seen),
-                    "last": last_seen.isoformat(),
+                    "last": last_seen.isoformat() if last_seen else None,
+                    "last_seen_source": last_seen_source,
+                    "sfm_reachable": sfm_reachable,
                     "fname": e.last_file,
                     "deployed": False,            # default
                     "note": "",
@@ -192,6 +299,7 @@ def emit_status_snapshot():
                         unit_data["status"] = paired_unit.get("status", "Missing")
                         unit_data["age"] = paired_unit.get("age", "N/A")
                         unit_data["last"] = paired_unit.get("last")
+                        unit_data["last_seen_source"] = paired_unit.get("last_seen_source", "none")
                         unit_data["derived_from"] = paired_unit_id
 
         # Separate buckets for UI

templates/partials/active_table.html

@@ -36,7 +36,14 @@
             </div>
 
             <!-- Age -->
-            <div class="text-right flex-shrink-0">
+            <div class="text-right flex-shrink-0 flex items-center gap-2">
+                {% if unit.last_seen_source == 'sfm' %}
+                <span class="text-[10px] uppercase tracking-wider px-1.5 py-0.5 rounded bg-seismo-orange/10 text-seismo-orange font-semibold"
+                      title="Status sourced from SFM event forwards (primary)">SFM</span>
+                {% elif unit.last_seen_source == 'heartbeat' %}
+                <span class="text-[10px] uppercase tracking-wider px-1.5 py-0.5 rounded bg-gray-100 dark:bg-gray-600 text-gray-500 dark:text-gray-300"
+                      title="Status sourced from watcher heartbeat (backup)">HB</span>
+                {% endif %}
                 <span class="text-sm {% if unit.status == 'Missing' %}text-red-600 dark:text-red-400 font-semibold{% elif unit.status == 'Pending' %}text-yellow-600 dark:text-yellow-400{% else %}text-gray-500 dark:text-gray-400{% endif %}">
                     {{ unit.age }}
                 </span>