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>

frontend/src/components/timeline/Timeline.tsx

@@ -3,6 +3,7 @@ import { useVirtualizer } from '@tanstack/react-virtual'
 import { usePhotoStore } from '../../store/photoStore'
 import { PhotoThumbnail } from './PhotoThumbnail'
 import { usePhotosQuery } from '../../hooks/usePhotosQuery'
+import { useActiveHeapMembers } from '../../hooks/useActiveHeapMembersQuery'
 import type { Photo } from '../../types/photo'
 
 export function Timeline() {
@@ -48,6 +49,11 @@ export function Timeline() {
   // they share one cache entry, regardless of filter state.
   const { data: photos = [], isLoading } = usePhotosQuery()
 
+  // Membership in the active heap (for the basket affordance). Subscribed
+  // once at this level so we don't have hundreds of thumbnails each
+  // subscribing to the same query.
+  const { memberIds: activeHeapMembers } = useActiveHeapMembers()
+
   // Group photos into rows for grid layout
   const rows = useMemo(() => {
     const result: Photo[][] = []
@@ -212,6 +218,7 @@ export function Timeline() {
                       photo={photo}
                       size={thumbnailSize}
                       isSelected={selectedPhotos.includes(photo.id)}
+                      isInActiveHeap={activeHeapMembers.has(photo.id)}
                       onClick={(e) => {
                         if (e.shiftKey && lastSelectedIndex !== null) {
                           selectRange(globalIndex)