clean(presentation): strip comments, kill dead code, extract constants

- remove all doc/inline comments
- delete unused ApiError variants (AuthRequired, NotImplemented) and helpers (internal, not_implemented)
- prefix lifetime-only AppState fields with _ to suppress dead_code warning
- extract magic numbers: TICK_INTERVAL_SECS, EXPIRY_THRESHOLD_HOURS, POLL_INTERVAL_SECS, EVENT_BUS_CAPACITY, DEFAULT_ACTIVITY_LIMIT, DEFAULT_SEARCH_LIMIT, etc
- replace inline serde_json::json! in sync_status with typed SyncStatusEntry
- fix mid-file import in schedule handler
- fix extractors: strip_prefix instead of magic offset 6
- fix unreachable pattern in wire_repositories with cfg guard
- use eq_ignore_ascii_case for content-type header check

crates/presentation/src/handlers/config.rs

@@ -1,5 +1,3 @@
-//! System configuration handler.
-
 use axum::Json;
 use axum::extract::State;
 
@@ -8,7 +6,8 @@ use api_types::{ConfigResponse, ProviderCapabilitiesResponse, ProviderInfo};
 use crate::errors::ApiError;
 use crate::state::AppState;
 
-/// GET /config — public system configuration
+const FALLBACK_STREAMING_PROTOCOL: &str = "direct_file";
+
 pub async fn get_config(
     State(state): State<AppState>,
 ) -> Result<Json<ConfigResponse>, ApiError> {
@@ -36,7 +35,7 @@ pub async fn get_config(
             tags: false,
             decade: false,
             search: false,
-            streaming_protocol: "direct_file".to_string(),
+            streaming_protocol: FALLBACK_STREAMING_PROTOCOL.to_string(),
             rescan: false,
             transcode: false,
         });