feat(backend): service cache Redis pour identifications

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

backend/app/services/redis_cache.py

@@ -0,0 +1,34 @@
+import hashlib
+import json
+import os
+from typing import Optional
+
+_client = None
+
+
+def _get_client():
+    global _client
+    if _client is None:
+        import redis
+        url = os.environ.get("REDIS_URL", "redis://localhost:6379")
+        _client = redis.from_url(url, decode_responses=True)
+    return _client
+
+
+def cache_key(image_bytes: bytes) -> str:
+    return f"identify:{hashlib.sha256(image_bytes).hexdigest()}"
+
+
+def get(image_bytes: bytes) -> Optional[list]:
+    try:
+        value = _get_client().get(cache_key(image_bytes))
+        return json.loads(value) if value else None
+    except Exception:
+        return None
+
+
+def set(image_bytes: bytes, results: list, ttl: int = 604800) -> None:
+    try:
+        _get_client().setex(cache_key(image_bytes), ttl, json.dumps(results))
+    except Exception:
+        pass  # cache indisponible → silencieux