harden(ft,auth,net): cap transfers/frames, evict stale SRP, distrust XFF

M1: enforce the declared transfer size (clamped to MAX_SIZE) on chunk
receipt in both the Rust and Python clients — a malicious sender can no
longer grow the receive buffer unboundedly.
M2: only honor X-Forwarded-For when TRUST_PROXY is set, so a direct
client can't spoof a source IP to dodge the per-IP rate limiter.
M3: evict unverified SRP sessions after a 60s TTL on each new handshake,
preventing half-finished auths from exhausting memory.
M4: drop WS frames larger than 256 KB before they hit the store or
broadcast, bounding per-message memory and flood blast radius.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd_chat/server/helpers.py

@@ -1,3 +1,4 @@
+import os
 import time
 from collections import defaultdict
 from datetime import datetime, timezone
@@ -10,9 +11,16 @@ def utcnow() -> datetime:
     return datetime.now(timezone.utc)
 
 
+# Only honor X-Forwarded-For when explicitly told we sit behind a trusted proxy
+# (TRUST_PROXY=1). Otherwise a direct client can spoof the header to forge a
+# source IP and dodge the per-IP rate limiter, so we use the real peer address.
+_TRUST_PROXY = os.environ.get("TRUST_PROXY", "").lower() in ("1", "true", "yes")
+
+
 def get_client_ip(request: Request) -> str:
-    if forwarded := request.headers.get("x-forwarded-for"):
-        return forwarded.split(",")[0].strip()
+    if _TRUST_PROXY:
+        if forwarded := request.headers.get("x-forwarded-for"):
+            return forwarded.split(",")[0].strip()
     return request.ip