feat(chat): Socket.IO 双端通道、消息持久化与幂等

访客与客服分别鉴权接入，消息带客户端 ID 去重，重发只产生一条业务记录。

backend/app/services/public_rate_limit.py

@@ -0,0 +1,70 @@
+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+Fixed-window per-key rate limiting for public unauthenticated endpoints.
+Redis-backed when available (shared across workers, like the socket rate
+limiter); per-process in-memory fallback otherwise.
+"""
+
+import time
+from threading import Lock
+
+from app.core.logger import get_logger
+
+logger = get_logger(__name__)
+
+# Prune expired windows once the fallback map exceeds this many keys, so
+# attacker-rotated keys can't grow memory without bound.
+_LOCAL_PRUNE_THRESHOLD = 10_000
+
+_local_counts: dict = {}  # key -> (count, window_start, window_seconds)
+_local_lock = Lock()
+
+
+def _redis_client():
+    try:
+        from app.services.socket_rate_limit import redis_client
+        return redis_client
+    except Exception:
+        return None
+
+
+def allow_request(key: str, limit: int, window_seconds: int) -> bool:
+    """True if `key` may make another request in the current fixed window."""
+    client = _redis_client()
+    if client:
+        try:
+            redis_key = f"public_rl:{key}:{window_seconds}"
+            # SET NX EX guarantees the key always carries a TTL before any
+            # INCR — an INCR-then-EXPIRE pair could leave an immortal counter
+            # if the EXPIRE step was lost.
+            client.set(redis_key, 0, nx=True, ex=window_seconds)
+            return client.incr(redis_key) <= limit
+        except Exception as e:
+            logger.warning(f"Redis rate limit failed, using local fallback: {e}")
+    now = time.monotonic()
+    with _local_lock:
+        if len(_local_counts) > _LOCAL_PRUNE_THRESHOLD:
+            expired = [
+                k for k, (_, start, window) in _local_counts.items()
+                if now - start >= window
+            ]
+            for k in expired:
+                del _local_counts[k]
+        count, window_start, _ = _local_counts.get(key, (0, now, window_seconds))
+        if now - window_start >= window_seconds:
+            count, window_start = 0, now
+        _local_counts[key] = (count + 1, window_start, window_seconds)
+        return count + 1 <= limit