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/local-files/src/config.rs

@@ -1,13 +1,8 @@
 use std::path::PathBuf;
 
-/// Configuration for the local files media provider.
 pub struct LocalFilesConfig {
-    /// Root directory containing video files. All files are served relative to this.
     pub root_dir: PathBuf,
-    /// Public base URL of this API server, used to build stream URLs.
     pub base_url: String,
-    /// Directory for FFmpeg HLS transcode cache. `None` disables transcoding.
     pub transcode_dir: Option<PathBuf>,
-    /// How long (hours) to keep transcode cache entries. Passed to TranscodeManager.
     pub cleanup_ttl_hours: u32,
 }