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/ai/insight_generator.rs

@@ -1187,6 +1187,7 @@ impl InsightGenerator {
 
         // 11. Store in database
         let insight = InsertPhotoInsight {
+            library_id: crate::libraries::PRIMARY_LIBRARY_ID,
             file_path: file_path.to_string(),
             title,
             summary,
@@ -2031,6 +2032,7 @@ Return ONLY the summary, nothing else."#,
         // Upsert a photo link so this entity is associated with this photo
         let link = InsertEntityPhotoLink {
             entity_id: subject_entity_id,
+            library_id: crate::libraries::PRIMARY_LIBRARY_ID,
             file_path: file_path.to_string(),
             role: photo_role,
         };
@@ -2742,6 +2744,7 @@ Return ONLY the summary, nothing else."#,
 
         // 15. Store insight (returns the persisted row including its new id)
         let insight = InsertPhotoInsight {
+            library_id: crate::libraries::PRIMARY_LIBRARY_ID,
             file_path: file_path.to_string(),
             title,
             summary: final_content,