fix(db): /db/units surfaces events-only serials too

Previous query_units() only joined on ach_sessions, which is created
exclusively by the live ACH server.  The BW-importer path
(/db/import/blastware_file → WaveformStore.save_imported_bw →
SeismoDb.insert_events) populates `events` but never creates an
ach_sessions row.  Consequence: every serial whose events flowed in
through the series3-watcher forwarder was invisible to
/db/units (and therefore to the SFM webapp's fleet overview / units
list), even though the events were correctly populated in the
events table with proper serial attribution.

Rewrite query_units() to aggregate from BOTH tables and union the
serials:
  - total_events / last_event_at  come from `events` (every ingest path)
  - last_session_at / total_monitor_entries / total_sessions
                                  come from `ach_sessions` (ACH-only),
                                  0 when no sessions exist for the serial
  - last_seen = max(last_event_at, last_session_at)

Verified on the user's actual prod DB after the
repair_unknown_serials run: /db/units now returns 24 serials instead
of 2.  All 3,257 watcher-forwarded events become visible in the
fleet overview without any further DB surgery.

sfm/database.py

@@ -564,21 +564,79 @@ class SeismoDb:
 
     def query_units(self) -> list[dict]:
         """
-        Return one row per known serial with summary stats:
+        Return one row per known serial with summary stats.
-        last_seen, total_events, total_monitor_entries.
+
+        Aggregates from BOTH source tables:
+          - `events`        — populated by every ingest path
+                              (live ACH, /db/import/blastware_file
+                              from the series3-watcher forwarder, etc.)
+          - `ach_sessions`  — only populated by the live ACH server;
+                              empty for events that came in via the
+                              BW-importer route.
+
+        Earlier this method only joined on `ach_sessions`, which made
+        watcher-forwarded units invisible to the SFM webapp's fleet
+        overview even though their events were correctly populated in
+        `events`.  Now we union the two and surface every serial that
+        has activity in either table.
+
+        Fields:
+          serial                — unit serial number (e.g. "BE11529")
+          last_seen             — most recent of MAX(events.timestamp)
+                                  and MAX(ach_sessions.session_time)
+          total_events          — COUNT(*) from `events` (the
+                                  authoritative count regardless of
+                                  ingest path)
+          total_monitor_entries — from `ach_sessions`, 0 when absent
+          total_sessions        — COUNT(*) from `ach_sessions`, 0 when absent
         """
         with self._connect() as conn:
-            rows = conn.execute(
+            event_stats = {
-                """
+                row["serial"]: row
-                SELECT
+                for row in conn.execute(
-                    s.serial,
-                    MAX(s.session_time)  AS last_seen,
-                    SUM(s.events_downloaded)  AS total_events,
-                    SUM(s.monitor_entries)    AS total_monitor_entries,
-                    COUNT(*)                  AS total_sessions
-                FROM ach_sessions s
-                GROUP BY s.serial
-                ORDER BY last_seen DESC
                     """
+                    SELECT serial,
+                           MAX(timestamp) AS last_event_at,
+                           COUNT(*)        AS total_events
+                      FROM events
+                     GROUP BY serial
+                    """,
                 ).fetchall()
-        return [dict(r) for r in rows]
+            }
+            session_stats = {
+                row["serial"]: row
+                for row in conn.execute(
+                    """
+                    SELECT serial,
+                           MAX(session_time)         AS last_session_at,
+                           SUM(monitor_entries)      AS total_monitor_entries,
+                           COUNT(*)                  AS total_sessions
+                      FROM ach_sessions
+                     GROUP BY serial
+                    """,
+                ).fetchall()
+            }
+
+        all_serials = set(event_stats) | set(session_stats)
+        units = []
+        for serial in all_serials:
+            e = event_stats.get(serial)
+            s = session_stats.get(serial)
+            last_event_at   = e["last_event_at"]   if e else None
+            last_session_at = s["last_session_at"] if s else None
+            # Prefer whichever timestamp is more recent
+            last_seen = max(
+                (t for t in (last_event_at, last_session_at) if t),
+                default=None,
+            )
+            units.append({
+                "serial":                serial,
+                "last_seen":             last_seen,
+                "total_events":          e["total_events"]          if e else 0,
+                "total_monitor_entries": s["total_monitor_entries"] if s else 0,
+                "total_sessions":        s["total_sessions"]        if s else 0,
+            })
+
+        # Sort by last_seen desc; serials with no timestamp at all sink to the bottom.
+        units.sort(key=lambda u: u.get("last_seen") or "", reverse=True)
+        return units