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/lib.rs

@@ -1,9 +1,3 @@
-//! Jellyfin media provider adapter.
-//!
-//! Implements [`domain::ports::IMediaProvider`] by talking to the Jellyfin HTTP API.
-//! The domain never sees Jellyfin-specific types -- this module translates
-//! between Jellyfin's API model and the domain's abstract `MediaItem`/`MediaFilter`.
-
 mod config;
 mod mapping;
 mod models;