feat: multi-library foundation (schema + libraries module)

Adds a `libraries` registry table and threads library_id through
per-instance metadata tables (image_exif, photo_insights,
entity_photo_links, video_preview_clips). File-path columns renamed to
rel_path to make the relative-to-root semantics explicit. Adds
content_hash + size_bytes on image_exif to support future hash-keyed
thumbnail/HLS dedup. Tags and favorites stay library-agnostic so they
share across libraries by rel_path.

Behavior is unchanged: a single primary library (id=1) is seeded from
BASE_PATH on first boot; all handlers and DAOs route through it as a
transitional shim until the API gains a library query param.

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

src/testhelpers.rs

@@ -98,6 +98,7 @@ impl PreviewDao for TestPreviewDao {
             file_path_val.to_string(),
             VideoPreviewClip {
                 id: *id,
+                library_id: crate::libraries::PRIMARY_LIBRARY_ID,
                 file_path: file_path_val.to_string(),
                 status: status_val.to_string(),
                 duration_seconds: None,