fix: quota pending sign-ins and registrations per address, keep young clients, write the store off the loop (SEC-09)

docs/security.md

@@ -98,7 +98,7 @@ Denial of service
 | Large request bodies | SDK body limit on the MCP endpoint and SDK OAuth routes; upload cap checked before decoding | `server.py` (`max_request_body_size`), `gitea/encoding.py` (consent route: SEC-01) |
 | Large or hostile responses | 16 MiB text cap, `MAX_BINARY_BYTES`, 64 KiB error bodies, JSON depth 100, item and character limits | `gitea/executor.py` |
 | Request floods | Sliding window per IP and per token hash, bounded key sets, rate-limit events audited once per window | `rate_limit.py`, `transports.py` (`ForwardedHeadersMiddleware`) |
-| Unbounded OAuth state | Caps on clients (500), pending sign-ins (1024), codes (1024), sessions per login (50); store pruning | `oauth/provider.py`, `oauth/store.py` (flush by floods: SEC-09) |
+| Unbounded OAuth state | Caps on clients (500), pending sign-ins (1024), codes (1024), sessions per login (50); per client address at most 20 pending sign-ins and 20 registrations per hour; clients younger than the pending window are never evicted; store pruning; the store write (fsync) runs in a worker thread | `oauth/provider.py`, `oauth/store.py` |
 
 Elevation of privilege