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/migrations/007_add_expiry.sql

@@ -0,0 +1,2 @@
+ALTER TABLE deliveries ADD COLUMN expires_at INTEGER;
+CREATE INDEX idx_deliveries_expires ON deliveries(expires_at) WHERE expires_at IS NOT NULL;