cleanup: strip comments, extract constants, DRY shared helpers across adapters + infra-wiring

- strip all comments except WHY workaround notes (3 remain)
- remove all #[allow(dead_code)]; fix via _prefix rename
- extract named constants: JWT time units, token types, default config values, jellyfin fallback bitrate
- DRY: move serialize_enum_as_string, content_type_str, parse_content_type, parse_genres_blob to adapter-common
- sqlite+postgres library.rs use shared helpers instead of local copies
- sqlite+postgres channel.rs use shared serialize_enum_as_string
- remove dead `let _ = ext` in scanner.rs

crates/adapters/postgres/src/settings.rs

@@ -1,5 +1,3 @@
-//! PostgreSQL adapter for app settings (AppSettingsRepository).
-
 use async_trait::async_trait;
 use sqlx::PgPool;