feat(exif): extract capture date and surface as sortable taken_at field

Adds EXIF date extraction during indexing so photos can be sorted by when
they were actually taken, not just when the file was modified on disk.

- Add `taken_at` (nullable TEXT, ISO 8601) column to the images table via
  ensure_column migration and a new idx_images_taken_at index
- Populate taken_at in build_record via extract_exif_date, which tries
  DateTimeOriginal → DateTimeDigitized → DateTime tags and rejects all-zero
  sentinel dates written by uninitialised cameras
- upsert_image uses taken_at = excluded.taken_at so a re-indexed file gets
  fresh EXIF; stale dates from replaced files are not preserved
- Add taken_asc / taken_desc sort using COALESCE(taken_at, created_at) so
  images without EXIF gracefully fall back to their creation date
- Surface as "Taken: newest / oldest" in the Toolbar sort dropdown
- Shift map_image_row column indices after the new taken_at (index 10)

src/components/Toolbar.tsx

@@ -5,6 +5,8 @@ import { tileSizeForZoom, useGalleryStore, SortOrder, MediaFilter, SearchCommand
 const BASE_SORT_OPTIONS: { value: SortOrder; label: string }[] = [
   { value: "date_desc", label: "Newest first" },
   { value: "date_asc", label: "Oldest first" },
+  { value: "taken_desc", label: "Taken: newest" },
+  { value: "taken_asc", label: "Taken: oldest" },
   { value: "name_asc", label: "Name A–Z" },
   { value: "name_desc", label: "Name Z–A" },
   { value: "rating_desc", label: "Highest rated" },