feat: add distributed rate limiting with sliding window algorithm

- RateLimiter trait with check_rate(key, config) -> RateResult
- InMemoryRateLimiter: DashMap-based sliding window log per key
- RateLimitConfig: configurable max_requests and window duration
- RateResult: allowed/remaining/retry_after_secs for Retry-After headers
- Lazy GC of expired entries (every 60s)
- Thread-safe concurrent access via DashMap
- 5 unit tests: limit enforcement, independent keys, remaining counter, concurrency

crates/quicproquo-server/src/domain/mod.rs

@@ -17,4 +17,5 @@ pub mod groups;
 pub mod p2p;
 pub mod account;
 pub mod moderation;
+pub mod rate_limit;
 pub mod recovery;