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-client/src/client/rpc.rs

@@ -226,6 +226,16 @@ pub async fn enqueue(
     client: &node_service::Client,
     recipient_key: &[u8],
     payload: &[u8],
+) -> anyhow::Result<u64> {
+    enqueue_with_ttl(client, recipient_key, payload, None).await
+}
+
+/// Enqueue with an optional TTL (seconds). 0 or None means no expiry.
+pub async fn enqueue_with_ttl(
+    client: &node_service::Client,
+    recipient_key: &[u8],
+    payload: &[u8],
+    ttl_secs: Option<u32>,
 ) -> anyhow::Result<u64> {
     let client = client.clone();
     let recipient_key = recipient_key.to_vec();
@@ -243,6 +253,9 @@ pub async fn enqueue(
                     p.set_payload(&payload);
                     p.set_channel_id(&[]);
                     p.set_version(1);
+                    if let Some(ttl) = ttl_secs {
+                        p.set_ttl_secs(ttl);
+                    }
                     let mut auth = p.reborrow().init_auth();
                     set_auth(&mut auth)?;
                 }
@@ -831,6 +844,32 @@ pub async fn download_blob_chunk(
     Ok((chunk, total_size, mime_type))
 }
 
+/// Delete the authenticated user's account on the server.
+/// Requires an identity-bound session (OPAQUE login).
+pub async fn delete_account(
+    client: &node_service::Client,
+) -> anyhow::Result<bool> {
+    let mut req = client.delete_account_request();
+    {
+        let mut p = req.get();
+        let mut auth = p.reborrow().init_auth();
+        set_auth(&mut auth)?;
+    }
+
+    let resp = req
+        .send()
+        .promise
+        .await
+        .context("delete_account RPC failed")?;
+
+    let success = resp
+        .get()
+        .context("delete_account: bad response")?
+        .get_success();
+
+    Ok(success)
+}
+
 /// Return the current Unix timestamp in milliseconds.
 pub fn current_timestamp_ms() -> u64 {
     std::time::SystemTime::now()