refactor: unify Pick with active heap membership

Pick and add-to-heap were two ways of saying "I want to keep this
one". Merging them: P now toggles the selection's membership in the
active heap. The is_picked flag goes away (orphaned in the DB the
same way is_trashed was).

Backend
- Drop is_picked from PhotoBase / PhotoUpdate / PhotoResponse and
  from the photos list filter param.
- Drop the is_picked Column from the Photo model (DB column stays
  on legacy installs but is no longer read or written).
- Drop the bulk action 'pick' branch.
- New GET /heaps/{id}/photo_ids returns just the flat string list.
  Used by the frontend for fast client-side membership lookups
  without fetching full photo records.

Frontend
- New hooks/useActiveHeapMembersQuery.ts → returns
  { activeHeap, memberIds: Set<string> }. Subscribes once at the
  Timeline level and passes a derived isInActiveHeap bool down to
  each PhotoThumbnail (avoids hundreds of thumbnails subscribing
  to the same query).
- PhotoThumbnail: replaces the old check-icon Pick affordance with
  a clear basket badge in the bottom-right corner — a small filled
  pick-colour pill containing a ShoppingBasket icon — visible only
  when the photo belongs to the active heap.
- P shortcut (useKeyboardShortcuts) now toggles membership: if every
  selected photo is already a member, it removes them; otherwise it
  adds the missing ones. T binding removed (P fully replaces it).
- RightSidebar Pick button is now a Pick / Picked toggle bound to
  the active heap. Disabled with a hint when no heap is active.
  Shows the heap name in its title attr.
- filterStore drops 'picked' and 'unflagged' from FlagFilter.
  FilterBar's flag dropdown is now just Any / Discarded.
- LeftSidebar drops the "Flagged" virtual node (it just set
  flag=picked, which no longer exists).
- KeyboardHints: P → "Pick → heap".
- Photo TS type drops is_picked.

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

backend/app/models/photos.py

@@ -55,10 +55,10 @@ class Photo(Base):
     user_notes = Column(Text)
     rating = Column(Integer, default=0)  # 0-5 stars
     color_label = Column(String)  # 'red'|'orange'|'yellow'|'green'|'blue'|'purple'|NULL
-    is_picked = Column(Boolean, default=False)
     # Note: is_rejected was merged into is_discarded (a single soft "discarded"
-    # concept). The DB column may still exist on legacy installs but is no
-    # longer read or written.
+    # concept). is_picked was unified with active-heap membership — picking a
+    # photo just means adding it to the active heap. Both DB columns may still
+    # exist on legacy installs but are no longer read or written.
     
     # Duplicate detection
     is_duplicate = Column(Boolean, default=False)