feat(locations): drag-to-reorder + three-dot kebab menu on cards

Project location cards now reorderable via drag-and-drop, and the
four inline action buttons (Unassign/Edit/Remove/Delete) collapse into
a single three-dot kebab menu — much cleaner card layout, especially
for projects with many locations.

Data
- MonitoringLocation.sort_order: nullable Integer, default 0.
  Migration `migrate_add_location_sort_order.py` adds the column and
  seeds existing rows with sort_order = alphabetical index per project
  (so the post-migration display order matches what operators see
  today — no surprise reordering).
- get_project_locations + locations-json: ORDER BY sort_order, name.
- Location-create: assigns max(sort_order) + 1 so new locations land
  at the END of the list rather than being interleaved alphabetically.

Reorder endpoint
- POST /api/projects/{p}/locations/reorder
  Body: { location_ids: [uuid, uuid, ...] }
  Validates: all ids belong to this project; raises 404 on missing.
  Applies 0-indexed sort_order matching the provided order.

UI changes (templates/partials/projects/location_list.html)
- Active cards get a draggable="true" attribute + native HTML5
  drag/drop handlers.  Drop reorders the DOM immediately, then posts
  the new order to the reorder endpoint.  Drop-zone visual feedback
  (orange ring on hover, opacity on source during drag).
- Six-dot drag handle icon on the left of each active card; whole
  card body is the drag source but the handle is the visual cue.
- Right side: small Assign pill (only shown when unassigned) +
  three-dot kebab menu containing Unassign/Edit/Remove/Delete.
  Click ⋮ to toggle; click outside or Escape to close.  Only one
  menu open at a time.
- Removed locations are NOT draggable (their order is historical) and
  keep their existing Restore button visible.

The card also shows "{N} events" instead of "Sessions: N" when the
location_type is vibration AND the backend passes event_count in
the payload — which lands in commit 2 of this redesign.

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

backend/models.py

@@ -243,6 +243,12 @@ class MonitoringLocation(Base):
     removed_at     = Column(DateTime, nullable=True)
     removal_reason = Column(Text, nullable=True)
 
+    # Display order within the project's location list.  Operators can
+    # drag-and-drop to reorder cards on the project detail page.  Lower
+    # values render first; ties fall back to name (alphabetical).  Seeded
+    # to alphabetical-index on migration; new locations get max+1.
+    sort_order = Column(Integer, default=0, nullable=False)
+
     created_at = Column(DateTime, default=datetime.utcnow)
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)