feat(infra): add Redis service and client for Phase 0 [FASE-0.1]

Add Redis 7 to Docker Compose with healthcheck and persistence, separate logical DBs for blacklist and cache, singleton redis client helpers, and unit tests with fakeredis.

backend/app/infrastructure/redis_client.py

@@ -1,17 +1,22 @@
-"""Redis client singleton.
+"""Redis client factories.
 
-Provides a lazily-initialised Redis connection that is reused across
-the application.  The connection URL is read from ``settings.REDIS_URL``.
+``settings.REDIS_URL`` selects the default logical database (usually ``0``).
+Token blacklist and application cache use separate logical DBs on the same
+Redis instance (``REDIS_TOKEN_BLACKLIST_DB``, ``REDIS_CACHE_DB``) so keys never
+collide and TTL policies can differ per workload.
 
 Usage::
 
-    from app.infrastructure.redis_client import get_redis
+    from app.infrastructure.redis_client import get_redis, get_redis_blacklist
 
-    r = get_redis()
-    r.set("key", "value", ex=300)
+    get_redis().set("key", "value", ex=300)
+    get_redis_blacklist().setex("blacklist:…", ttl, "1")
 """
 
+from __future__ import annotations
+
 import logging
+from urllib.parse import urlparse, urlunparse
 
 import redis
 
@@ -19,16 +24,43 @@ from app.config import settings
 
 logger = logging.getLogger(__name__)
 
-_redis_client: redis.Redis | None = None
+_clients: dict[str, redis.Redis] = {}
+
+
+def _redis_url_with_db(base_url: str, db_index: int) -> str:
+    """Return *base_url* with its path replaced by ``/{db_index}``."""
+    parsed = urlparse(base_url)
+    path = f"/{db_index}"
+    return urlunparse(
+        (parsed.scheme, parsed.netloc, path, "", "", ""),
+    )
+
+
+def _get_client(url: str) -> redis.Redis:
+    if url not in _clients:
+        _clients[url] = redis.from_url(url, decode_responses=True)
+        logger.info("Redis client connected to %s", url)
+    return _clients[url]
 
 
 def get_redis() -> redis.Redis:
-    """Return a shared Redis client, creating it on first call."""
-    global _redis_client
-    if _redis_client is None:
-        _redis_client = redis.from_url(
-            settings.REDIS_URL,
-            decode_responses=True,
-        )
-        logger.info("Redis client connected to %s", settings.REDIS_URL)
-    return _redis_client
+    """Default Redis connection (URL from ``settings.REDIS_URL``)."""
+    return _get_client(settings.REDIS_URL)
+
+
+def get_redis_blacklist() -> redis.Redis:
+    """Redis DB used for JWT revocation (``jti`` keys with TTL)."""
+    url = _redis_url_with_db(
+        settings.REDIS_URL,
+        settings.REDIS_TOKEN_BLACKLIST_DB,
+    )
+    return _get_client(url)
+
+
+def get_redis_cache() -> redis.Redis:
+    """Redis DB reserved for shared cache (scores, queues, etc.)."""
+    url = _redis_url_with_db(
+        settings.REDIS_URL,
+        settings.REDIS_CACHE_DB,
+    )
+    return _get_client(url)