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/config.py

@@ -24,6 +24,9 @@ class Settings(BaseSettings):
 
     # ── Redis ─────────────────────────────────────────────────────────
     REDIS_URL: str = "redis://redis:6379/0"
+    # Logical DB indices on the same Redis instance (PATH in URL is overridden).
+    REDIS_TOKEN_BLACKLIST_DB: int = 1
+    REDIS_CACHE_DB: int = 2
 
     # ── CORS ─────────────────────────────────────────────────────────
     # Comma-separated list of allowed origins, or a JSON array.