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/main.rs

@@ -1191,6 +1191,7 @@ fn main() -> std::io::Result<()> {
                 .service(ai::get_available_models_handler)
                 .service(ai::rate_insight_handler)
                 .service(ai::export_training_data_handler)
+                .service(libraries::list_libraries)
                 .add_feature(add_tag_services::<_, SqliteTagDao>)
                 .add_feature(knowledge::add_knowledge_services::<_, SqliteKnowledgeDao>)
                 .app_data(app_data.clone())