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/layout/LeftSidebar.tsx

@@ -1,17 +1,16 @@
 import { useState } from 'react'
-import { 
+import {
   ChevronRight,
   ChevronDown,
   Folder,
   Image,
   Calendar,
   Star,
-  Flag,
   Trash2,
   Plus,
   MoreHorizontal,
   HardDrive,
-  RefreshCw
+  RefreshCw,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { AddSourceFolderDialog } from '../dialogs/AddSourceFolderDialog'
@@ -52,10 +51,6 @@ export function LeftSidebar() {
         clearAllFilters()
         setRatingMin(1)
         break
-      case 'flagged':
-        clearAllFilters()
-        setFlag('picked')
-        break
       case 'discarded':
         clearAllFilters()
         setFlag('discarded')
@@ -138,7 +133,6 @@ export function LeftSidebar() {
         { id: 'all-photos', label: 'All Photos', icon: <Image className="h-4 w-4" />, count: 0 },
         { id: 'by-date', label: 'By Date', icon: <Calendar className="h-4 w-4" /> },
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: 0 },
-        { id: 'flagged', label: 'Flagged', icon: <Flag className="h-4 w-4" />, count: 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: 0 },
       ],
     },