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/hooks/useActiveHeapMembersQuery.ts

@@ -0,0 +1,39 @@
+import { useMemo } from 'react'
+import { useQuery } from '@tanstack/react-query'
+import { useHeapsQuery } from './useHeapsQuery'
+import { heaps as heapsApi } from '../services/api'
+
+const EMPTY_SET: ReadonlySet<string> = new Set()
+
+/**
+ * Returns the photo ids that belong to the active heap as a Set, plus the
+ * active heap itself. Used by PhotoThumbnail to render the basket affordance
+ * and by the P shortcut to decide between add vs remove.
+ *
+ * If no heap is active, the Set is empty (and shared across renders).
+ */
+export function useActiveHeapMembers(): {
+  activeHeap: ReturnType<typeof useHeapsQuery>['data'] extends (infer T)[] | undefined
+    ? T | null
+    : never
+  memberIds: ReadonlySet<string>
+} {
+  const { data: heaps } = useHeapsQuery()
+  const activeHeap = heaps?.find((h) => h.is_active) ?? null
+
+  const { data: ids } = useQuery({
+    queryKey: ['heap-photo-ids', activeHeap?.id],
+    queryFn: () => heapsApi.photoIds(activeHeap!.id),
+    enabled: !!activeHeap,
+    staleTime: 30_000,
+  })
+
+  const memberIds = useMemo(
+    () => (ids ? new Set(ids) : EMPTY_SET),
+    [ids]
+  )
+
+  return { activeHeap: activeHeap as any, memberIds }
+}
+
+export const ACTIVE_HEAP_MEMBERS_QUERY_KEY_PREFIX = ['heap-photo-ids'] as const