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.

schemas/node.capnp

@@ -23,7 +23,7 @@ interface NodeService {
   # Returns the monotonically increasing per-inbox sequence number assigned to this message,
   # plus a cryptographic delivery proof (96 bytes: 32-byte SHA-256 preimage || 64-byte Ed25519
   # signature). Old clients that do not read deliveryProof are unaffected (Cap'n Proto optional).
-  enqueue @2 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16, auth :Auth) -> (seq :UInt64, deliveryProof :Data);
+  enqueue @2 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16, auth :Auth, ttlSecs :UInt32) -> (seq :UInt64, deliveryProof :Data);
 
   # Fetch and drain all queued payloads for the recipient.
   # limit: max number of messages to return (0 = fetch all).
@@ -80,7 +80,7 @@ interface NodeService {
   fetchHybridKeys @16 (identityKeys :List(Data), auth :Auth) -> (keys :List(Data));
 
   # Enqueue the same payload to multiple recipients in a single round-trip.
-  batchEnqueue @17 (recipientKeys :List(Data), payload :Data, channelId :Data, version :UInt16, auth :Auth) -> (seqs :List(UInt64));
+  batchEnqueue @17 (recipientKeys :List(Data), payload :Data, channelId :Data, version :UInt16, auth :Auth, ttlSecs :UInt32) -> (seqs :List(UInt64));
 
   # Create a 1:1 channel between the caller and the given peer. Returns a 16-byte channelId (UUID).
   # Both members can enqueue/fetch for this channel; recipientKey must be the other member.
@@ -116,6 +116,13 @@ interface NodeService {
   # length    : maximum number of bytes to return (capped at 256 KB).
   # Returns the requested chunk, the total blob size, and its MIME type.
   downloadBlob @22 (auth :Auth, blobId :Data, offset :UInt64, length :UInt32) -> (chunk :Data, totalSize :UInt64, mimeType :Text);
+
+  # Permanently delete the authenticated user's account and all associated data.
+  # Requires an identity-bound session (OPAQUE login). Removes user record,
+  # identity keys, key packages, hybrid keys, queued messages, and channel memberships.
+  # A tombstone entry is added to the KT log for auditability.
+  # All active sessions for the identity are invalidated.
+  deleteAccount @23 (auth :Auth) -> (success :Bool);
 }
 
 struct Auth {