feat(arr): offer subtitle providers to the API

Provider credentials are bootstrap config and never reach the database
(DESIGN.md §10), so which providers exist is settled once at startup;
which of them a search runs is the `providers_enabled` row the API reads
per request. OpenSubtitles.com cannot be called without a registered API
key, so without one it is not offered at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/arr-daemon/src/main.rs

@@ -144,7 +144,13 @@ async fn run() -> Result<(), Error> {
     if let Some(tmdb_url) = config.tmdb_url {
         upstreams = upstreams.with_tmdb_url(tmdb_url);
     }
-    let state = AppState::new(upstreams)?.with_database(database.clone());
+    let state = AppState::new(upstreams)?
+        .with_database(database.clone())
+        .with_subtitle_providers(subtitle_providers(
+            config.opensubtitles_api_key,
+            config.opensubtitles_username,
+            config.opensubtitles_password,
+        ));
 
     let app = arr_api::router(state.clone())
         .merge(arr_compat::router(compat))
@@ -443,3 +449,42 @@ async fn shutdown() {
 
     tracing::info!("shutting down");
 }
+
+/// The subtitle providers this deployment can reach (DESIGN.md §15).
+///
+/// Credentials are bootstrap config and never reach the database (§10), so
+/// which providers *exist* is decided here, once, at startup; which of them a
+/// search *runs* is the `providers_enabled` setting the API reads per
+/// request. OpenSubtitles.com needs a registered API key to be called at all,
+/// so without one it is not offered; Podnapisi is anonymous and always is.
+///
+/// Translation backends are not wired: none is implemented yet (#191-#193),
+/// and `AppState` defaults to an empty registry, which makes a manual
+/// translation answer "no such engine" rather than fail obscurely.
+fn subtitle_providers(
+    opensubtitles_api_key: Option<String>,
+    username: Option<String>,
+    password: Option<String>,
+) -> Vec<std::sync::Arc<dyn arr_subs::Provider>> {
+    let mut providers: Vec<std::sync::Arc<dyn arr_subs::Provider>> = Vec::new();
+
+    match arr_subs::Podnapisi::new() {
+        Ok(podnapisi) => providers.push(std::sync::Arc::new(podnapisi)),
+        Err(error) => tracing::warn!(%error, "Podnapisi not available"),
+    }
+
+    let Some(api_key) = opensubtitles_api_key else {
+        tracing::info!("no OpenSubtitles.com API key configured; that provider is off");
+        return providers;
+    };
+    match arr_subs::OpenSubtitles::new(arr_subs::OpenSubtitlesConfig {
+        api_key,
+        username,
+        password,
+    }) {
+        Ok(opensubtitles) => providers.push(std::sync::Arc::new(opensubtitles)),
+        Err(error) => tracing::warn!(%error, "OpenSubtitles.com not available"),
+    }
+
+    providers
+}