feat: add content_hash backfill + register every media file

Adds blake3 content hashing as the basis for derivative dedup
(thumbnails, HLS) across libraries. Computed inline by the watcher on
ingest and by a new `backfill_hashes` binary for historical rows.

Key changes:
- `content_hash` and `size_bytes` are now populated on new image_exif
  rows; a new ExifDao surface (`get_rows_missing_hash`,
  `backfill_content_hash`, `find_by_content_hash`) supports backfill and
  future hash-keyed lookups.
- The watcher now registers every image/video in image_exif, not just
  files with parseable EXIF. EXIF becomes optional enrichment; videos
  and other non-EXIF files still get a hashed row. This also makes
  DB-indexed sort/filter cover the full library.
- `/image` thumbnail serve dual-looks up hash-keyed path first, then
  falls back to the legacy mirrored layout.
- Upload flow accepts `?library=` query param + hashes uploaded files.
- Store_exif logs the underlying Diesel error on insert failure so
  constraint violations surface instead of hiding behind a generic
  InsertError.
- New migration normalizes rel_path separators to forward slash across
  all tables, deduplicating any rows that collide after normalization.
  Fixes spurious UNIQUE violations from mixed backslash/forward-slash
  paths on Windows ingest.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/files.rs

@@ -1360,6 +1360,33 @@ mod tests {
         ) -> Result<Vec<(String, f64, f64, Option<i64>)>, DbError> {
             todo!()
         }
+
+        fn get_rows_missing_hash(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _limit: i64,
+        ) -> Result<Vec<(i32, String)>, DbError> {
+            Ok(Vec::new())
+        }
+
+        fn backfill_content_hash(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+            _rel_path: &str,
+            _hash: &str,
+            _size_bytes: i64,
+        ) -> Result<(), DbError> {
+            Ok(())
+        }
+
+        fn find_by_content_hash(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _hash: &str,
+        ) -> Result<Option<crate::database::models::ImageExif>, DbError> {
+            Ok(None)
+        }
     }
 
     mod api {