chore(v1.1.7): fix clippy --all-targets warnings

Clears the workspace under `clippy --all-targets --all-features
-D warnings`. Four were pre-existing at v1.1.6 HEAD (latent finding,
see HANDBACK): double_must_use on realtime_router, map_unwrap_or in
pubsub_service, redundant_closure in topic_repo, needless_raw_string in
a subscriber-token test. The rest are v1.1.7 nits (needless_borrow +
semicolon in the dead-letter / realtime-migration code).

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

crates/orchestrator-core/src/realtime_api.rs

@@ -80,7 +80,6 @@ pub fn heartbeat_secs_from_env() -> u64 {
 }
 
 /// Router for the realtime SSE surface. Merged at the router root.
-#[must_use]
 pub fn realtime_router(state: RealtimeState) -> Router {
     Router::new()
         .route("/realtime/topics/{topic}", get(sse_topic))