feat: add GET /libraries and library query param plumbing

New `/libraries` endpoint returns configured libraries so clients can
discover them. `FilesRequest` and `MemoriesRequest` gain an optional
`library` param (accepts name or numeric id). Unknown values are
rejected with 400; absent values span all libraries. `/memories`
now scopes its filesystem walk + EXIF query to the resolved library.
`MemoryItem` carries `library_id` so union-mode clients can render a
per-item source badge.

Behavior is unchanged in single-library mode: omitting `library` still
returns results from the primary library, which is the only one
configured until a second row is added to the libraries table.

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

src/data/mod.rs

@@ -155,6 +155,10 @@ pub struct FilesRequest {
     // Pagination parameters (optional - backward compatible)
     pub limit: Option<i64>,
     pub offset: Option<i64>,
+
+    /// Optional library filter. Accepts a library id (e.g. "1") or name
+    /// (e.g. "main"). When omitted, results span all libraries.
+    pub library: Option<String>,
 }
 
 #[derive(Copy, Clone, Deserialize, PartialEq, Debug)]