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/providers.rs

@@ -1,5 +1,3 @@
-//! Provider configuration CRUD handlers.
-
 use axum::Json;
 use axum::extract::{Path, State};
 
@@ -12,7 +10,6 @@ use crate::errors::ApiError;
 use crate::extractors::AdminUser;
 use crate::state::AppState;
 
-/// GET /admin/providers
 pub async fn list_providers(
     State(state): State<AppState>,
     AdminUser(_user): AdminUser,
@@ -27,7 +24,6 @@ pub async fn list_providers(
     ))
 }
 
-/// GET /admin/providers/:id
 pub async fn get_provider(
     State(state): State<AppState>,
     AdminUser(_user): AdminUser,
@@ -40,7 +36,6 @@ pub async fn get_provider(
     Ok(Json(ProviderConfigResponse::from(provider)))
 }
 
-/// PUT /admin/providers/:id
 pub async fn upsert_provider(
     State(state): State<AppState>,
     AdminUser(_user): AdminUser,
@@ -60,7 +55,6 @@ pub async fn upsert_provider(
     Ok(Json(serde_json::json!({"status": "ok"})))
 }
 
-/// DELETE /admin/providers/:id
 pub async fn delete_provider(
     State(state): State<AppState>,
     AdminUser(_user): AdminUser,