feat(auth): move JWT blacklist to Redis with TTL [FASE-0.2]

Revoke tokens by jti in a dedicated Redis DB, honor TTL from JWT exp on logout, reject revoked tokens in get_current_user, and add FakeRedis-backed API tests.

backend/app/auth.py

@@ -80,11 +80,11 @@ def blacklist_token(jti: str, exp: float) -> None:
     to ``exp - now`` so the key vanishes when the token would have expired
     naturally.
     """
-    from app.infrastructure.redis_client import get_redis
+    from app.infrastructure.redis_client import get_redis_blacklist
 
     ttl = max(int(exp - datetime.now(timezone.utc).timestamp()), 1)
     try:
-        r = get_redis()
+        r = get_redis_blacklist()
         r.setex(f"{_BLACKLIST_PREFIX}{jti}", ttl, "1")
     except Exception:
         logger.warning("Failed to blacklist token %s in Redis", jti, exc_info=True)
@@ -92,10 +92,10 @@ def blacklist_token(jti: str, exp: float) -> None:
 
 def is_token_blacklisted(jti: str) -> bool:
     """Return ``True`` if *jti* has been revoked (exists in Redis)."""
-    from app.infrastructure.redis_client import get_redis
+    from app.infrastructure.redis_client import get_redis_blacklist
 
     try:
-        r = get_redis()
+        r = get_redis_blacklist()
         return r.exists(f"{_BLACKLIST_PREFIX}{jti}") > 0
     except Exception:
         logger.warning("Failed to check blacklist for %s in Redis", jti, exc_info=True)