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/error_codes.rs

@@ -30,6 +30,7 @@ pub const E024_BLOB_TOO_LARGE: &str = "E024";
 pub const E025_BLOB_HASH_LENGTH: &str = "E025";
 pub const E026_BLOB_HASH_MISMATCH: &str = "E026";
 pub const E027_BLOB_NOT_FOUND: &str = "E027";
+pub const E028_ACCOUNT_DELETION_FAILED: &str = "E028";
 
 /// Build a `capnp::Error::failed()` with the structured code prefix.
 pub fn coded_error(code: &str, msg: impl std::fmt::Display) -> capnp::Error {