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/store/filterStore.ts

@@ -2,7 +2,7 @@ import { create } from 'zustand'
 
 export type MediaType = 'photo' | 'video' | 'raw' | 'heic'
 export type ColorLabel = 'red' | 'orange' | 'yellow' | 'green' | 'blue' | 'purple'
-export type FlagFilter = 'any' | 'picked' | 'discarded' | 'unflagged'
+export type FlagFilter = 'any' | 'discarded'
 
 export interface FilterState {
   q: string
@@ -82,9 +82,7 @@ export function filtersToParams(f: FilterState): Record<string, string | number>
   if (f.mediaTypes.length > 0) params.media_type = f.mediaTypes.join(',')
   if (f.ratingMin > 0) params.rating_min = f.ratingMin
   if (f.colorLabel) params.color_label = f.colorLabel
-  if (f.flag === 'picked') params.is_picked = 'true'
-  else if (f.flag === 'discarded') params.is_discarded = 'true'
-  else if (f.flag === 'unflagged') params.is_picked = 'false'
+  if (f.flag === 'discarded') params.is_discarded = 'true'
   if (f.heapId) params.heap_id = f.heapId
   return params
 }