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-core/src/group.rs

@@ -611,6 +611,11 @@ impl GroupMember {
         self.hybrid
     }
 
+    /// Return the current MLS epoch, or `None` if no group is active.
+    pub fn epoch(&self) -> Option<u64> {
+        self.group.as_ref().map(|g| g.epoch().as_u64())
+    }
+
     /// Return a reference to the MLS group, if active.
     pub fn group_ref(&self) -> Option<&MlsGroup> {
         self.group.as_ref()