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  badge next to historical attributions whose location is no longer
  active.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/models.py

@@ -235,6 +235,14 @@ class MonitoringLocation(Base):
     # For vibration: {"ground_type": "bedrock", "depth": "10m"}
     location_metadata = Column(Text, nullable=True)
 
+    # Soft-removal: NULL means active.  When set, the location is hidden from
+    # active surfaces (assign dropdowns, calendar, scheduler, dashboard
+    # vibration summary) but historical events generated before this time
+    # still attribute to it.  Mirrors the closed-state pattern used by
+    # UnitAssignment.assigned_until.
+    removed_at     = Column(DateTime, nullable=True)
+    removal_reason = Column(Text, nullable=True)
+
     created_at = Column(DateTime, default=datetime.utcnow)
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)