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/auth/src/password.rs

@@ -1,10 +1,6 @@
-//! Password hashing adapter using the `password-auth` crate.
-
 use domain::errors::DomainResult;
 use domain::ports::AuthService;
 
-/// Concrete `AuthService` implementation backed by `password-auth`
-/// (Argon2id by default).
 pub struct PasswordAuthService;
 
 impl AuthService for PasswordAuthService {