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/jellyfin/src/config.rs

@@ -1,10 +1,6 @@
-/// Connection details for a single Jellyfin instance.
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
 pub struct JellyfinConfig {
-    /// e.g. `"http://192.168.1.10:8096"` -- no trailing slash.
     pub base_url: String,
-    /// Jellyfin API key (Settings -> API Keys).
     pub api_key: String,
-    /// The Jellyfin user ID used for library browsing.
     pub user_id: String,
 }