Harden auth and security controls with session auth and docs

backend/app/services/rate_limiter.py

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+"""Redis-backed fixed-window rate limiter helpers for sensitive API operations."""
+
+import time
+
+from redis.exceptions import RedisError
+
+from app.worker.queue import get_redis
+
+
+def _rate_limit_key(*, scope: str, subject: str, window_id: int) -> str:
+    """Builds a stable Redis key for one scope, subject, and fixed time window."""
+
+    return f"dcm:rate-limit:{scope}:{subject}:{window_id}"
+
+
+def increment_rate_limit(
+    *,
+    scope: str,
+    subject: str,
+    limit: int,
+    window_seconds: int = 60,
+) -> tuple[int, int]:
+    """Increments one rate bucket and returns current count with configured limit."""
+
+    bounded_limit = max(0, int(limit))
+    if bounded_limit == 0:
+        return (0, 0)
+
+    bounded_window = max(1, int(window_seconds))
+    current_window = int(time.time() // bounded_window)
+    key = _rate_limit_key(scope=scope, subject=subject, window_id=current_window)
+
+    redis_client = get_redis()
+    try:
+        pipeline = redis_client.pipeline(transaction=True)
+        pipeline.incr(key, 1)
+        pipeline.expire(key, bounded_window + 5)
+        count_value, _ = pipeline.execute()
+    except RedisError as error:
+        raise RuntimeError("Rate limiter backend unavailable") from error
+
+    return (int(count_value), bounded_limit)