feat(sfm): per-unit event history with attribution + Unattributed bucket

Phase 2 of the SFM integration.  Adds a "SFM Events" section to the
seismograph unit detail page (/unit/{id}).  Every event SFM has for the
serial is shown, with each event annotated by which project/location
assignment window it falls into.  Events outside every assignment window
get the "⚠ Unattributed" badge plus a "<N>d before/after <nearest location>"
hint — that's the operator's signal that backdating an assignment (Phase 1
edit-pencil) will absorb the orphan events.

Backend:
- backend/services/sfm_events.py: new events_for_unit() helper.  Fetches
  all events for the serial via SFM /db/events (one call, ceiling 5000),
  loads every UnitAssignment for the unit + resolves MonitoringLocation +
  Project names, then annotates each event with attribution or
  nearest_assignment (signed delta_days).  Bucket filter: all /
  attributed / unattributed.  Stats always reflect the full event set so
  the "Unattributed" KPI tile is meaningful regardless of which bucket
  is being viewed.

- backend/routers/units.py: new GET /api/units/{unit_id}/events with
  bucket / date-range / false_trigger / limit query params.  404s on
  unknown unit_id; returns an empty payload for non-seismograph
  device_types so the page can render the section conditionally.

Frontend (templates/unit_detail.html):
- New "SFM Events" section between "Deployment History" and "Timeline",
  styled to match the existing card pattern (border-t divider, same
  heading weight).
- Hidden by default; revealed only when currentUnit.device_type ===
  'seismograph' after the unit data loads.
- Four KPI tiles: Total Events / Unattributed (highlighted amber when
  > 0) / Peak PVS / Last Event.
- Filters: Bucket (all|attributed|unattributed), From/To, False
  Triggers, Limit, + Refresh.
- Event table with Attribution column.  Attributed rows link to the
  project/location detail page; unattributed rows are tinted amber
  and show "<N>d before/after <nearest location>" with a link to the
  nearest location.
- Empty-state copy varies by bucket: e.g. unattributed-with-zero shows
  "✅ All events for this unit are attributed to a project/location".

Verified end-to-end against BE11529 (81 events total, 24 attributed,
57 unattributed — all 57 unattributed events emitted within hours of
the assignment start, which means backdating the assignment by a day
would attribute every one of them).

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

backend/routers/units.py

@@ -1,7 +1,7 @@
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy.orm import Session
 from datetime import datetime
-from typing import Dict, Any
+from typing import Dict, Any, Optional
 
 from backend.database import get_db
 from backend.services.snapshot import emit_status_snapshot
@@ -72,3 +72,67 @@ def get_unit_by_id(unit_id: str, db: Session = Depends(get_db)):
         "slm_serial_number": unit.slm_serial_number,
         "deployed_with_modem_id": unit.deployed_with_modem_id
     }
+
+
+@router.get("/units/{unit_id}/events")
+async def get_unit_events(
+    unit_id: str,
+    bucket: str = Query("all", regex="^(all|attributed|unattributed)$"),
+    from_dt: Optional[datetime] = Query(None),
+    to_dt: Optional[datetime] = Query(None),
+    false_trigger: Optional[bool] = Query(None),
+    limit: int = Query(500, ge=1, le=5000),
+    db: Session = Depends(get_db),
+):
+    """
+    Return SFM events for a single unit, annotated with assignment attribution.
+
+    Each event includes an `attribution` object pointing at the project/location
+    it falls into (or null if outside every assignment window).  Unattributed
+    events also carry a `nearest_assignment` field with `delta_days` so the
+    operator can see how far off the nearest assignment is — useful for
+    deciding whether to backdate the assignment to absorb the event.
+
+    Bucket filter:
+      - all (default): every event
+      - attributed:    only events inside an assignment window
+      - unattributed:  only orphan events (the diagnostic bucket)
+
+    Non-seismograph units return an empty events list.  The route does not
+    404 for SLMs/modems so the unit detail page can render the section
+    conditionally without depending on the response shape.
+    """
+    unit = db.query(RosterUnit).filter_by(id=unit_id).first()
+    if not unit:
+        raise HTTPException(status_code=404, detail=f"Unit {unit_id} not found")
+
+    if unit.device_type != "seismograph":
+        return {
+            "events":            [],
+            "count":              0,
+            "stats": {
+                "event_count":         0,
+                "unattributed_count":  0,
+                "peak_pvs":            None,
+                "peak_pvs_at":         None,
+                "peak_pvs_serial":     None,
+                "last_event":          None,
+                "false_trigger_count": 0,
+            },
+            "assignments_total":  0,
+            "device_type":        unit.device_type,
+        }
+
+    from backend.services.sfm_events import events_for_unit
+
+    result = await events_for_unit(
+        db,
+        unit_id,
+        bucket=bucket,
+        from_dt=from_dt,
+        to_dt=to_dt,
+        false_trigger=false_trigger,
+        limit=limit,
+    )
+    result["device_type"] = unit.device_type
+    return result