feat(scaling): enable horizontal scaling for API and worker pods

- Add unauthenticated /api/diagnostic/healthz/live and /healthz/ready
  probe endpoints for Kubernetes liveness/readiness checks
- Separate Celery Beat into dedicated beat service in docker-compose.yaml
- Remove container_name from api and worker services to allow scaling
- Create Helm beat-deployment.yaml for standalone Beat scheduler pod
- Remove -B flag from worker-deployment.yaml so workers can scale safely
- Add beat section and fix probe paths in Helm values.yaml
- Add tests for the new probe endpoints

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_diagnostic.py

@@ -5,6 +5,86 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 
+@pytest.mark.unit
+class TestLivenessProbe:
+    """Tests for GET /api/diagnostic/healthz/live (unauthenticated)."""
+
+    def test_liveness_returns_200(self, client):
+        """Liveness probe always returns 200 OK."""
+        response = client.get("/api/diagnostic/healthz/live")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["status"] == "ok"
+
+
+@pytest.mark.unit
+class TestReadinessProbe:
+    """Tests for GET /api/diagnostic/healthz/ready (unauthenticated)."""
+
+    def test_readiness_returns_200_when_all_ok(self, client):
+        """Readiness probe returns 200 when database and Redis are reachable."""
+        with (
+            patch("app.api.diagnostic.engine") as mock_engine,
+            patch("app.api.diagnostic.redis_lib") as mock_redis,
+        ):
+            mock_conn = MagicMock()
+            mock_engine.connect.return_value.__enter__ = MagicMock(return_value=mock_conn)
+            mock_engine.connect.return_value.__exit__ = MagicMock(return_value=False)
+            mock_redis_inst = MagicMock()
+            mock_redis.from_url.return_value = mock_redis_inst
+
+            response = client.get("/api/diagnostic/healthz/ready")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["status"] == "ready"
+        assert data["checks"]["database"]["status"] == "ok"
+
+    def test_readiness_returns_503_when_database_fails(self, client):
+        """Readiness probe returns 503 when database is unreachable."""
+        with (
+            patch("app.api.diagnostic.engine") as mock_engine,
+            patch("app.api.diagnostic.redis_lib") as mock_redis,
+        ):
+            mock_engine.connect.side_effect = Exception("DB unavailable")
+            mock_redis_inst = MagicMock()
+            mock_redis.from_url.return_value = mock_redis_inst
+
+            response = client.get("/api/diagnostic/healthz/ready")
+
+        assert response.status_code == 503
+        data = response.json()
+        assert data["status"] == "not_ready"
+        assert data["checks"]["database"]["status"] == "error"
+
+    def test_readiness_returns_200_when_redis_fails(self, client):
+        """Readiness remains 200 when only Redis is down (non-critical)."""
+        with (
+            patch("app.api.diagnostic.engine") as mock_engine,
+            patch("app.api.diagnostic.redis_lib") as mock_redis,
+        ):
+            mock_conn = MagicMock()
+            mock_engine.connect.return_value.__enter__ = MagicMock(return_value=mock_conn)
+            mock_engine.connect.return_value.__exit__ = MagicMock(return_value=False)
+            mock_redis.from_url.return_value = MagicMock()
+            mock_redis.from_url.return_value.ping.side_effect = Exception("Connection refused")
+
+            response = client.get("/api/diagnostic/healthz/ready")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["status"] == "ready"
+        assert data["checks"]["redis"]["status"] == "error"
+
+    def test_readiness_contains_checks_keys(self, client):
+        """Readiness response always contains database and redis checks."""
+        response = client.get("/api/diagnostic/healthz/ready")
+        data = response.json()
+        assert "checks" in data
+        assert "database" in data["checks"]
+        assert "redis" in data["checks"]
+
+
 @pytest.mark.unit
 class TestHealthEndpoint:
     """Tests for GET /api/diagnostic/health endpoint."""