feat: server chains (client -> A -> B -> internet), audit log, users list, csv export

Chains are real Xray hops: a chain-<code> inbound + outbound + routing rule on the
entry node and a relay-<code> client on the exit node, reconciled by sync_node with
config test, port check, rollback on a failed restart and a per-node lock.
Admin API gets chains CRUD/probe/check, node latency, audit log (middleware, no request
bodies), users list with subscription counters and a csv export that neutralises formulas.

backup.py

@@ -123,5 +123,8 @@ def restore_backup(data: bytes) -> dict:
     os.chmod(tmp_db_path, 0o600)
     os.replace(tmp_db_path, DB_PATH)
 
+    import db
+    db.init_db()
+
     _prune_old_safety_copies()
     return {"restored_env": restored_env, "safety_copy": safety_copy}