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/libraries.rs

@@ -1,11 +1,14 @@
+use actix_web::{HttpResponse, Responder, get, web::Data};
 use chrono::Utc;
 use diesel::prelude::*;
 use diesel::sqlite::SqliteConnection;
 use log::{info, warn};
 use std::path::{Path, PathBuf};
 
+use crate::data::Claims;
 use crate::database::models::{InsertLibrary, LibraryRow};
 use crate::database::schema::libraries;
+use crate::state::AppState;
 
 /// Id of the primary library row seeded by the multi-library migration.
 /// Used as the default `library_id` during the Phase 2 transitional shim,
@@ -116,6 +119,43 @@ pub fn seed_or_patch_from_env(conn: &mut SqliteConnection, base_path: &str) {
     }
 }
 
+/// Resolve a library request parameter (accepts numeric id as string or name)
+/// against the configured libraries. Returns `Ok(None)` when the param is
+/// absent, meaning "span all libraries". Returns `Err` when a value is
+/// provided but does not match any library.
+pub fn resolve_library_param<'a>(
+    state: &'a AppState,
+    param: Option<&str>,
+) -> Result<Option<&'a Library>, String> {
+    let Some(raw) = param.map(str::trim).filter(|s| !s.is_empty()) else {
+        return Ok(None);
+    };
+
+    if let Ok(id) = raw.parse::<i32>() {
+        return state
+            .library_by_id(id)
+            .map(Some)
+            .ok_or_else(|| format!("unknown library id: {}", id));
+    }
+
+    state
+        .library_by_name(raw)
+        .map(Some)
+        .ok_or_else(|| format!("unknown library name: {}", raw))
+}
+
+#[derive(serde::Serialize)]
+pub struct LibrariesResponse {
+    pub libraries: Vec<Library>,
+}
+
+#[get("/libraries")]
+pub async fn list_libraries(_claims: Claims, app_state: Data<AppState>) -> impl Responder {
+    HttpResponse::Ok().json(LibrariesResponse {
+        libraries: app_state.libraries.clone(),
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;