fix(server): clippy fixes for Task 4 (CI green)

Task 4 (bfcdbb5) compiles and passes all 32 tests on the build host; only
clippy -D warnings blocked CI. Fixed the two denials:
- rate_limit.rs: converted a dangling /// doc block (no documented item) to //
  to clear clippy::empty_line_after_doc_comments
- db/events.rs: #[allow(dead_code)] on CONNECTION_REJECTED_EXPIRED_CODE and
  _CANCELLED_CODE (not-yet-wired audit-event constants), matching the file's
  existing STREAMING_STOPPED pattern; TODO comments note the rejection-event wiring

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/middleware/rate_limit.rs

@@ -325,12 +325,12 @@ fn too_many_requests(detail: &str, error_code: &str) -> Response {
 // Axum middleware functions (one per protected route)
 // ============================================================================
 
-/// Selects which limiter from [`RateLimitState`] a middleware uses.
-///
-/// Each protected route gets its own `from_fn_with_state` middleware pointing at
-/// the matching limiter; keeping them as distinct functions avoids threading an
-/// extra "which limiter" parameter through the layer and keeps the wiring in
-/// `main.rs` self-documenting.
+// Selects which limiter from `RateLimitState` a middleware uses.
+//
+// Each protected route gets its own `from_fn_with_state` middleware pointing at
+// the matching limiter; keeping them as distinct functions avoids threading an
+// extra "which limiter" parameter through the layer and keeps the wiring in
+// `main.rs` self-documenting.
 
 /// Rate-limit middleware for `POST /api/auth/login`.
 pub async fn login_rate_limit(