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/data/mod.rs

@@ -239,6 +239,8 @@ pub struct MetadataResponse {
     pub size: u64,
     pub exif: Option<ExifMetadata>,
     pub filename_date: Option<i64>, // Date extracted from filename
+    pub library_id: Option<i32>,
+    pub library_name: Option<String>,
 }
 
 impl From<fs::Metadata> for MetadataResponse {
@@ -255,6 +257,8 @@ impl From<fs::Metadata> for MetadataResponse {
             size: metadata.len(),
             exif: None,
             filename_date: None, // Will be set in endpoint handler
+            library_id: None,
+            library_name: None,
         }
     }
 }