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-tauri/Cargo.toml

@@ -50,6 +50,7 @@ ort = { version = "2.0.0-rc.12", default-features = false, features = ["std", "n
 ureq = { version = "3.3.0", default-features = false, features = ["native-tls"] }
 zip = { version = "4.6.1", default-features = false, features = ["deflate"] }
 csv = "1"
+kamadak-exif = "0.5"
 tauri-plugin-notification = "2"
 
 # ── Dev-mode performance ────────────────────────────────────────────────────