feat: wire up storage latency metrics, uptime gauge, and config timeouts

Instrument DeliveryService (enqueue, fetch) and KeyService
(key_package_upload, key_package_fetch) with storage latency histogram
recording. Add periodic uptime gauge task (every 15s). Log effective
rpc_timeout_secs, storage_timeout_secs, and webtransport_listen at
startup to eliminate dead_code warnings on EffectiveConfig fields.

crates/quicprochat-server/src/domain/keys.rs

@@ -32,8 +32,10 @@ impl KeyService {
         }
 
         let fingerprint: Vec<u8> = Sha256::digest(&req.package).to_vec();
+        let start = std::time::Instant::now();
         self.store
             .upload_key_package(&req.identity_key, req.package)?;
+        crate::metrics::record_storage_latency("key_package_upload", start.elapsed());
 
         Ok(UploadKeyPackageResp { fingerprint })
     }
@@ -43,7 +45,9 @@ impl KeyService {
         req: FetchKeyPackageReq,
         _auth: &CallerAuth,
     ) -> Result<FetchKeyPackageResp, DomainError> {
+        let start = std::time::Instant::now();
         let package = self.store.fetch_key_package(&req.identity_key)?;
+        crate::metrics::record_storage_latency("key_package_fetch", start.elapsed());
         Ok(FetchKeyPackageResp {
             package: package.unwrap_or_default(),
         })