feat(locations): soft-remove monitoring locations without destroying history

When a client drops a location from scope mid-project (e.g. the office
half of a museum+office monitoring job), operators couldn't previously
mark it as no-longer-active without either deleting it (which would
orphan historical events) or leaving it in the active list looking
deployable.  Now there's a proper middle ground.

Data model
- MonitoringLocation gets two new nullable columns:
  - removed_at      — NULL means active; set means soft-removed
  - removal_reason  — optional operator note
  Migration: backend/migrate_add_location_removed.py (idempotent)

Endpoints
- POST /api/projects/{p}/locations/{l}/remove
    Body: { effective_date?: ISO-datetime, reason?: str }
    Side effects (cascade):
      1. Closes active UnitAssignment rows at this location
         (assigned_until = effective_date, status = "completed")
      2. Cancels pending ScheduledActions at this location
      3. Marks location.removed_at = effective_date
    Returns counts of assignments closed + actions cancelled.
- POST /api/projects/{p}/locations/{l}/restore
    Clears removed_at + removal_reason.  Does NOT auto-reopen
    assignments — operator creates new ones if resuming monitoring.

Active-surface filters
- locations-json defaults to active-only; pass include_removed=true
  for historical / reporting views.  Schedule modal dropdowns now
  exclude removed locations automatically.
- Metadata-backfill fuzzy matcher excludes removed locations from
  proposed targets (don't want backfill creating new assignments at
  decommissioned locations).
- Vibration-summary per_location rollup includes removed locations
  (so historical event totals stay accurate) but tags each with
  removed_at so the UI can show a badge.

UI
- Project detail page's Monitoring Locations section now splits into:
    Active locations (full card with Assign / Edit / Remove / Delete)
    Removed locations (collapsed <details>, greyed cards, Restore button,
                       shows removal date + reason)
- New per-card "Remove" button → opens confirmation modal explaining
  the cascade, with optional effective-date (defaults to now,
  backdateable) and reason fields.
- Unit detail's SFM Events attribution cell shows a small "removed"
  badge next to historical attributions whose location is no longer
  active.  Same pattern in vibration_summary's top-locations list.
- Soft-removal indicator surfaced through the events_for_unit
  attribution payload as location_removed_at.

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

backend/services/sfm_events.py

@@ -318,13 +318,18 @@ async def events_for_unit(
         loc = loc_map.get(a.location_id)
         proj = proj_map.get(a.project_id)
         return {
-            "assignment_id":  a.id,
-            "location_id":    a.location_id,
-            "location_name":  loc.name if loc else None,
-            "project_id":     a.project_id,
-            "project_name":   proj.name if proj else None,
-            "assigned_at":    _iso_utc(a.assigned_at),
-            "assigned_until": _iso_utc(a.assigned_until),
+            "assignment_id":      a.id,
+            "location_id":        a.location_id,
+            "location_name":      loc.name if loc else None,
+            # Soft-removal indicator so the UI can render a "(removed)"
+            # badge next to historical attributions whose location is no
+            # longer actively monitored.
+            "location_removed_at": (loc.removed_at.isoformat()
+                                    if loc and loc.removed_at else None),
+            "project_id":         a.project_id,
+            "project_name":       proj.name if proj else None,
+            "assigned_at":        _iso_utc(a.assigned_at),
+            "assigned_until":     _iso_utc(a.assigned_until),
         }
 
     # 2. Fetch all events for this serial in one shot.
@@ -515,6 +520,10 @@ async def vibration_summary_for_project(
             "event_count":   ec,
             "peak_pvs":      ev_peak,
             "last_event":    ev_last,
+            # Soft-removal state — UI can show a "(removed)" badge in the
+            # per-location list so operators see at a glance that a row's
+            # numbers are historical-only.
+            "removed_at":    loc.removed_at.isoformat() if loc.removed_at else None,
         })
 
     per_location.sort(key=lambda r: r["event_count"], reverse=True)