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/PhotoThumbnail.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useCallback, useRef } from 'react'
-import { Star, Check, Trash2, RefreshCw } from 'lucide-react'
+import { Star, ShoppingBasket, Trash2, RefreshCw, Check } from 'lucide-react'
 import clsx from 'clsx'
 import { photos as photosApi } from '../../services/api'
 import type { Photo } from '../../types/photo'
@@ -12,11 +12,20 @@ interface PhotoThumbnailProps {
   photo: Photo
   size: number
   isSelected: boolean
+  /** True when the photo belongs to the currently active heap. */
+  isInActiveHeap?: boolean
   onClick: (e: React.MouseEvent) => void
   onDoubleClick?: (e: React.MouseEvent) => void
 }
 
-export function PhotoThumbnail({ photo, size, isSelected, onClick, onDoubleClick }: PhotoThumbnailProps) {
+export function PhotoThumbnail({
+  photo,
+  size,
+  isSelected,
+  isInActiveHeap = false,
+  onClick,
+  onDoubleClick,
+}: PhotoThumbnailProps) {
   const [imageError, setImageError] = useState(false)
   const [imageLoaded, setImageLoaded] = useState(false)
   const [retryCount, setRetryCount] = useState(0)
@@ -167,9 +176,14 @@ export function PhotoThumbnail({ photo, size, isSelected, onClick, onDoubleClick
       )}
       
       {/* Flag Indicators */}
-      <div className="absolute bottom-1 right-1">
-        {photo.is_picked && (
-          <Check className="h-4 w-4 text-pick" />
+      <div className="absolute bottom-1 right-1 flex items-center gap-1">
+        {isInActiveHeap && (
+          <div
+            className="flex h-5 w-5 items-center justify-center rounded-full bg-pick text-white shadow-md"
+            title="In active heap"
+          >
+            <ShoppingBasket className="h-3 w-3" />
+          </div>
         )}
         {photo.is_discarded && (
           <Trash2 className="h-4 w-4 text-reject" />

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)