feat: heaps end-to-end with active heap and T shortcut

Adds the spec §6.10 heaps concept: named photo collections with a
single "active" target for fast keyboard adds. Uses a basket icon
(ShoppingBasket) to visually distinguish heaps from folders.

Backend (routers/heaps.py)
- Replaces the 27-line stub with full CRUD: list (with photo counts
  via a single LEFT JOIN), create, patch (rename + set active), delete.
- Add/remove photos endpoints with idempotent semantics: re-adding an
  existing member is a no-op, removing a non-member is a no-op.
- Setting is_active=true on one heap clears the flag on every other
  heap in a single UPDATE so we maintain the single-active invariant.
- routers/photos.py list endpoint now applies the heap_id filter via
  IN-subquery against heap_photos (it was a declared param but had
  no filter logic).

Frontend
- New hooks/useHeapsQuery.ts and useFilterUrlSync wires heap_id as
  another URL-persisted filter; usePhotosQuery threads it through.
- New components/heaps/HeapsPanel.tsx replaces the LeftSidebar Heaps
  stub. Shows the basket icon, photo counts, lets you create heaps
  inline, click to filter the timeline, set active via the target
  icon, and delete heaps.
- TopBar shows an "active heap" pill (basket + name) so the user
  always knows where the next T-press will land.
- KeyboardHints adds T → Add to heap.

T shortcut (useKeyboardShortcuts)
- Reads the active heap from the heaps query cache and the selection
  from the photo store at fire time. Adds the selected photos (or the
  active photo if nothing is selected) via POST /heaps/{id}/photos.
- Toasts:
  - "Added to {heap}: N photos (M already present)" on success
  - "No active heap" hint when none is set
  - "Nothing selected" hint when there's no selection or active photo

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/KeyboardHints.tsx

@@ -13,6 +13,7 @@ export function KeyboardHints() {
         { key: '1-5', action: 'Rate' },
         { key: 'P', action: 'Pick' },
         { key: 'X', action: 'Discard' },
+        { key: 'T', action: 'Add to heap' },
         { key: 'Space', action: 'Preview' },
         { key: 'Esc', action: 'Deselect' },
       ]