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  badge next to historical attributions whose location is no longer
  active.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/unit_detail.html

@@ -2286,12 +2286,17 @@ function _ueAttrCell(ev) {
     if (a) {
         const projLabel = _ueEsc(a.project_name || '—');
         const locLabel  = _ueEsc(a.location_name || '—');
+        // If the attributed location has since been soft-removed, badge
+        // it so operators see at a glance this is historical attribution.
+        const removedBadge = a.location_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" title="Location no longer actively monitored">removed</span>'
+            : '';
         return `<a href="/projects/${_ueEsc(a.project_id)}/nrl/${_ueEsc(a.location_id)}"
                    onclick="event.stopPropagation()"
                    class="text-seismo-orange hover:text-seismo-navy"
                    title="${projLabel} → ${locLabel}">
                     📍 ${locLabel}
-                </a>
+                </a>${removedBadge}
                 <div class="text-xs text-gray-500 dark:text-gray-400">${projLabel}</div>`;
     }
     const n = ev.nearest_assignment;