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.lock

@@ -3113,6 +3113,15 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "kamadak-exif"
+version = "0.5.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ef4fc70d0ab7e5b6bafa30216a6b48705ea964cdfc29c050f2412295eba58077"
+dependencies = [
+ "mutate_once",
+]
+
 [[package]]
 name = "keyboard-types"
 version = "0.7.0"
@@ -3510,6 +3519,12 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "mutate_once"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "13d2233c9842d08cfe13f9eac96e207ca6a2ea10b80259ebe8ad0268be27d2af"
+
 [[package]]
 name = "native-tls"
 version = "0.2.18"
@@ -4290,6 +4305,7 @@ dependencies = [
  "hf-hub",
  "hnsw_rs",
  "image",
+ "kamadak-exif",
  "log",
  "memmap2",
  "ort",