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/files.rs

@@ -235,8 +235,24 @@ pub async fn list_photos<TagD: TagDao, FS: FileSystemAccess>(
             )
             .to_string(),
         ),
+        KeyValue::new("library", req.library.clone().unwrap_or_default()),
     ]);
 
+    // Resolve the optional library filter. Unknown values return 400.
+    // For Phase 3 the filesystem walk still operates against a single
+    // library's root; Phase 4 introduces multi-root union scanning.
+    let library = match crate::libraries::resolve_library_param(
+        &app_state,
+        req.library.as_deref(),
+    ) {
+        Ok(lib) => lib,
+        Err(msg) => {
+            log::warn!("Rejecting /photos request: {}", msg);
+            return HttpResponse::BadRequest().body(msg);
+        }
+    };
+    let scoped_library = library.unwrap_or_else(|| app_state.primary_library());
+
     let span_context = opentelemetry::Context::current_with_span(span);
 
     // Check if EXIF filtering is requested