fix: resolve media across libraries for video, metadata, and insights

The /video/generate and /image/metadata handlers assumed files live under
the resolved library only, which broke when a mobile client passed no
library (union mode) but the file lived in a non-primary library. Both
now fall back to scanning every configured library for an existing file.

InsightGenerator held a single base_path, so vision-model loads and
filename-date fallbacks failed for non-primary libraries. It now takes
Vec<Library> and probes each root in resolve_full_path.

/image/metadata responses now carry library_id/library_name so the
mobile viewer can surface which library a file belongs to.

Thumbnail generation at startup is now spawned on a background thread
so the HTTP server can accept traffic while large libraries backfill.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/state.rs

@@ -174,7 +174,7 @@ impl Default for AppState {
             search_dao.clone(),
             tag_dao.clone(),
             knowledge_dao,
-            base_path.clone(),
+            libraries_vec.clone(),
         );
 
         // Ensure preview clips directory exists
@@ -245,6 +245,11 @@ impl AppState {
 
         // Initialize test InsightGenerator with all data sources
         let base_path_str = base_path.to_string_lossy().to_string();
+        let test_lib = Library {
+            id: crate::libraries::PRIMARY_LIBRARY_ID,
+            name: "main".to_string(),
+            root_path: base_path_str.clone(),
+        };
         let insight_generator = InsightGenerator::new(
             ollama.clone(),
             sms_client.clone(),
@@ -256,7 +261,7 @@ impl AppState {
             search_dao.clone(),
             tag_dao.clone(),
             knowledge_dao,
-            base_path_str.clone(),
+            vec![test_lib],
         );
 
         // Initialize test preview DAO