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>

migrations/2026-04-17-000100_normalize_path_separators/down.sql

@@ -0,0 +1,4 @@
+-- No-op: there's no sensible way to recover which rows originally used
+-- backslashes, and there's no reason to want backslashes back. The
+-- deleted duplicates are also gone.
+SELECT 1;

migrations/2026-04-17-000100_normalize_path_separators/up.sql

@@ -0,0 +1,85 @@
+-- Normalize `rel_path` columns to forward slashes. Windows ingest
+-- historically produced a mix of `\` and `/`, which broke lookups and
+-- caused spurious UNIQUE-constraint violations on re-registration.
+--
+-- SQLite enforces UNIQUE per-row during UPDATE, so we have to drop
+-- losing duplicates BEFORE normalizing. For each table that has a
+-- UNIQUE on rel_path, we delete rows whose normalized form already
+-- exists in canonical (forward-slash) form — keeping the existing
+-- forward-slash row as the survivor. Then a flat UPDATE finishes the
+-- job for remaining backslash rows.
+
+-- image_exif: UNIQUE(library_id, rel_path)
+DELETE FROM image_exif
+ WHERE rel_path LIKE '%\%'
+   AND EXISTS (
+       SELECT 1 FROM image_exif AS other
+        WHERE other.library_id = image_exif.library_id
+          AND other.rel_path = REPLACE(image_exif.rel_path, '\', '/')
+          AND other.id != image_exif.id
+   );
+UPDATE image_exif
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+-- favorites: UNIQUE(userid, rel_path)
+DELETE FROM favorites
+ WHERE rel_path LIKE '%\%'
+   AND EXISTS (
+       SELECT 1 FROM favorites AS other
+        WHERE other.userid = favorites.userid
+          AND other.rel_path = REPLACE(favorites.rel_path, '\', '/')
+          AND other.id != favorites.id
+   );
+UPDATE favorites
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+-- tagged_photo: UNIQUE(rel_path, tag_id)
+DELETE FROM tagged_photo
+ WHERE rel_path LIKE '%\%'
+   AND EXISTS (
+       SELECT 1 FROM tagged_photo AS other
+        WHERE other.tag_id = tagged_photo.tag_id
+          AND other.rel_path = REPLACE(tagged_photo.rel_path, '\', '/')
+          AND other.id != tagged_photo.id
+   );
+UPDATE tagged_photo
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+-- entity_photo_links: UNIQUE(entity_id, library_id, rel_path, role)
+DELETE FROM entity_photo_links
+ WHERE rel_path LIKE '%\%'
+   AND EXISTS (
+       SELECT 1 FROM entity_photo_links AS other
+        WHERE other.entity_id = entity_photo_links.entity_id
+          AND other.library_id = entity_photo_links.library_id
+          AND other.role = entity_photo_links.role
+          AND other.rel_path = REPLACE(entity_photo_links.rel_path, '\', '/')
+          AND other.id != entity_photo_links.id
+   );
+UPDATE entity_photo_links
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+-- video_preview_clips: UNIQUE(library_id, rel_path)
+DELETE FROM video_preview_clips
+ WHERE rel_path LIKE '%\%'
+   AND EXISTS (
+       SELECT 1 FROM video_preview_clips AS other
+        WHERE other.library_id = video_preview_clips.library_id
+          AND other.rel_path = REPLACE(video_preview_clips.rel_path, '\', '/')
+          AND other.id != video_preview_clips.id
+   );
+UPDATE video_preview_clips
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+-- photo_insights has no UNIQUE on rel_path (history table), so a plain
+-- normalize is safe.
+UPDATE photo_insights
+   SET rel_path = REPLACE(rel_path, '\', '/')
+ WHERE rel_path LIKE '%\%';
+
+ANALYZE;