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/tests/test_redis_client.py

@@ -0,0 +1,61 @@
+"""Redis URL helpers and blacklist DB wiring (FASE 0.1)."""
+
+import time
+import uuid
+from unittest.mock import MagicMock
+
+import pytest
+
+from app.config import settings
+from app.infrastructure import redis_client as rc
+
+
+@pytest.fixture
+def fakeredis():
+    import fakeredis as fr
+
+    return fr.FakeRedis(decode_responses=True)
+
+
+def test_redis_url_with_db_replaces_index():
+    assert (
+        rc._redis_url_with_db("redis://localhost:6379/0", 2)
+        == "redis://localhost:6379/2"
+    )
+
+
+def test_get_redis_blacklist_uses_configured_db(monkeypatch):
+    monkeypatch.setattr(rc, "_clients", {})
+    captured: list[str] = []
+
+    def fake_from_url(url, **kwargs):
+        captured.append(url)
+        m = MagicMock()
+        m.setex = MagicMock()
+        return m
+
+    monkeypatch.setattr("redis.from_url", fake_from_url)
+    monkeypatch.setattr(settings, "REDIS_URL", "redis://redis:6379/0")
+    monkeypatch.setattr(settings, "REDIS_TOKEN_BLACKLIST_DB", 9)
+
+    c = rc.get_redis_blacklist()
+    c.setex("k", 10, "v")
+    assert captured == ["redis://redis:6379/9"]
+    c.setex.assert_called_once_with("k", 10, "v")
+
+
+def test_redis_set_get_ttl(fakeredis, monkeypatch):
+    """Integration-style check against FakeRedis (no real server)."""
+    monkeypatch.setattr(rc, "_clients", {})
+    monkeypatch.setattr(
+        "app.infrastructure.redis_client.get_redis_blacklist",
+        lambda: fakeredis,
+    )
+    from app.auth import blacklist_token, is_token_blacklisted
+
+    jti = str(uuid.uuid4())
+    exp = time.time() + 3600
+    blacklist_token(jti, exp)
+    assert is_token_blacklisted(jti) is True
+    ttl = fakeredis.ttl(f"blacklist:{jti}")
+    assert 0 < ttl <= 3600