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/Cargo.toml

@@ -20,6 +20,7 @@ arr-indexer = { workspace = true }
 arr-meta = { workspace = true }
 arr-parse = { workspace = true }
 arr-probe = { workspace = true }
+arr-subs = { workspace = true }
 axum = { workspace = true }
 chrono = { workspace = true }
 include_dir = { workspace = true }