feat: Sprint 6 — disappearing messages, group info, account deletion

- Disappearing messages: ttlSecs param on enqueue/batchEnqueue RPCs,
  expires_at column (migration 007), server GC deletes expired messages,
  /disappear command with human-friendly duration parsing (30m, 1h, 1d)
- Group info: /group-info shows type, members, MLS epoch; /rename
  renames conversations; /members resolves usernames via resolveIdentity
- Account deletion: deleteAccount @23 RPC with transactional purge of
  all user data (deliveries, keys, channels), session invalidation,
  KT log preserved for auditability; /delete-account with confirmation
- Added epoch() accessor to GroupMember, enqueue_with_ttl client helper

All 35 server + 71 core + 14 E2E tests pass.

crates/quicproquo-server/src/federation/service.rs

@@ -54,7 +54,7 @@ impl federation_service::Server for FederationServiceImpl {
             return Promise::err(capnp::Error::failed("payload must not be empty".into()));
         }
 
-        let seq = match self.store.enqueue(&recipient_key, &channel_id, payload) {
+        let seq = match self.store.enqueue(&recipient_key, &channel_id, payload, None) {
             Ok(s) => s,
             Err(e) => return Promise::err(capnp::Error::failed(format!("store error: {e}"))),
         };
@@ -106,7 +106,7 @@ impl federation_service::Server for FederationServiceImpl {
                     format!("recipient_key[{i}] must be 32 bytes"),
                 ));
             }
-            let seq = match self.store.enqueue(&rk, &channel_id, payload.clone()) {
+            let seq = match self.store.enqueue(&rk, &channel_id, payload.clone(), None) {
                 Ok(s) => s,
                 Err(e) => return Promise::err(capnp::Error::failed(format!("store error: {e}"))),
             };