feat: include per-photo library id in /photos response

Adds a parallel `photo_libraries: Vec<i32>` array alongside `photos`
in `PhotosResponse` so clients can render per-thumbnail badges.
Populated with the scoped library id at the two main return sites;
left empty for `/favorites` since favorites are library-agnostic.

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

src/data/mod.rs

@@ -102,6 +102,12 @@ pub struct PhotosResponse {
     pub photos: Vec<String>,
     pub dirs: Vec<String>,
 
+    /// Library id for each entry in `photos`, same length and ordering.
+    /// Parallel array rather than an object per row to keep the payload
+    /// small and backwards-compatible with older clients.
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub photo_libraries: Vec<i32>,
+
     // Pagination metadata (only present when limit is set)
     #[serde(skip_serializing_if = "Option::is_none")]
     pub total_count: Option<i64>,