feat: add Redis-backed rate limiter with configurable backend

Add RedisRateLimiter using sorted-set sliding window alongside the
existing InMemoryRateLimiter. Users choose via RATE_LIMIT_BACKEND
(memory|redis) with RATE_LIMIT_REDIS_URL falling back to RQ_REDIS_URL.
Redis backend validates connectivity at startup and fails open on
transient errors during requests.

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

backend/app/core/rate_limit_backend.py

@@ -0,0 +1,12 @@
+"""Rate-limit backend selection enum."""
+
+from __future__ import annotations
+
+from enum import Enum
+
+
+class RateLimitBackend(str, Enum):
+    """Supported rate-limiting backends."""
+
+    MEMORY = "memory"
+    REDIS = "redis"