feat: make file watcher, thumbnails, and upload library-aware

`watch_files` and `create_thumbnails` now iterate every configured
library, tagging rows with the correct `library_id`. `process_new_files`
takes a `&Library` so InsertImageExif no longer hardcodes the primary
library. Upload accepts an optional `library` query param to pick a
target library; omitted still defaults to primary for backwards
compatibility.

Hash-keyed thumbnail/HLS storage with dual-lookup fallback is deferred
to Phase 5, where it's bundled with the content hash backfill that
actually makes the hash-keyed paths meaningful. Until hashes are
populated, the legacy mirrored layout is a no-op to change.

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

src/files.rs

@@ -1159,7 +1159,10 @@ impl Handler<RefreshThumbnailsMessage> for StreamActor {
         let tracer = global_tracer();
         let _ = tracer.start("RefreshThumbnailsMessage");
         info!("Refreshing thumbnails after upload");
-        create_thumbnails()
+        // The stub in lib.rs is a no-op; the real generation is driven by
+        // the file watcher tick in main.rs, which has access to the
+        // configured libraries.
+        create_thumbnails(&[])
     }
 }