Thumb orientation + library filter on /photos/exif

Two follow-ups on the same feature branch:

1. Bake EXIF orientation into generated thumbnails. The `image` crate
   doesn't apply Orientation on load, and `save_with_format(..Jpeg)`
   drops EXIF — so portrait phone shots ended up sideways in any client
   that displays the cached thumb directly (no EXIF tag for the browser
   to compensate from). New `exif::read_orientation` reads the tag
   cheaply (no full EXIF parse) and `exif::apply_orientation` does the
   rotate/flip via image's existing `rotate90/180/270` + `fliph/flipv`.
   Applied in both branches of `generate_image_thumbnail` (RAW embedded-
   JPEG path and the regular `image::open` path). Existing thumbnails
   in the cache are still wrong-orientation; wipe the thumb dir or run
   a one-off backfill once this lands.

2. Optional `library` query param on `/photos/exif`. Accepts numeric id
   or name (same shape as `/image?library=...`), resolved via the
   existing `resolve_library_param` helper so a bad value 400s before
   we touch the DAO. Filter is applied post-query in the handler
   rather than pushed into `query_by_exif` to keep the DAO trait
   (and its test mocks) unchanged. Cheap enough at typical library
   counts; can be moved into SQL later if it ever isn't.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/data/mod.rs

@@ -418,6 +418,10 @@ pub struct ExifBatchRequest {
     pub date_from: Option<i64>,
     /// Upper bound (inclusive). Same semantics as `date_to` on `/photos`.
     pub date_to: Option<i64>,
+    /// Restrict results to a single library by id. Omit (or "" / "all") for
+    /// union mode — the default. Filtered post-query in the handler so the
+    /// existing `query_by_exif` DAO trait stays untouched.
+    pub library: Option<String>,
 }
 
 #[derive(Deserialize)]