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  badge next to historical attributions whose location is no longer
  active.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>

templates/partials/projects/vibration_summary.html

@@ -60,6 +60,10 @@
                class="flex items-center justify-between py-1.5 px-3 rounded hover:bg-gray-50 dark:hover:bg-slate-700/50 transition-colors">
                 <span class="text-sm font-medium text-gray-900 dark:text-white truncate">
                     📍 {{ loc.location_name }}
+                    {% if loc.removed_at %}
+                    <span class="ml-1 text-[10px] uppercase tracking-wider px-1 py-0.5 rounded bg-gray-200 dark:bg-gray-700 text-gray-600 dark:text-gray-300 font-semibold align-middle"
+                          title="Location no longer actively monitored — events shown are historical">removed</span>
+                    {% endif %}
                 </span>
                 <span class="flex items-center gap-4 text-sm text-gray-600 dark:text-gray-400 whitespace-nowrap ml-3">
                     <span>{{ "{:,}".format(loc.event_count) }} event{{ '' if loc.event_count == 1 else 's' }}</span>