test: close coverage gaps and add frontend test infrastructure

Backend (516 tests, 94% coverage):
- Add azure_openai endpoint/deployment validation tests (config.py -> 100%)
- Add _total_conversations and _avg_turns direct tests (queries.py -> 100%)
- Add transformer edge cases: list content, string checkpoint, invalid JSON,
  malformed message graceful skip (transformer.py -> 93%)
- Add safety combined status_code+error_message interaction tests
- Fix ambiguous 200/422 assertion to strict 422
- Add E2E pagination shape assertions (total, page, per_page, row count)
- Fix ReplayPool mock to respect LIMIT/OFFSET params

Frontend (23 tests, vitest + happy-dom + @testing-library/react):
- Add vitest infrastructure with happy-dom environment
- Add api.ts tests: success, HTTP error, success=false, URL encoding
- Add DashboardPage tests: loading, data, error, empty states
- Add ReplayListPage tests: loading, empty, data, error, status badge classes
- Add ReplayPage tests: loading, steps, empty, error states

backend/tests/unit/test_config.py

@@ -89,3 +89,21 @@ class TestSettings:
                 anthropic_api_key="key",
                 openai_api_key="",
             )
+
+    def test_azure_openai_missing_endpoint_rejected(self) -> None:
+        with pytest.raises(ValueError, match="AZURE_OPENAI_ENDPOINT"):
+            _isolated_settings(
+                database_url="postgresql://x:x@localhost/db",
+                llm_provider="azure_openai",
+                azure_openai_api_key="key",
+                azure_openai_deployment="my-deploy",
+            )
+
+    def test_azure_openai_missing_deployment_rejected(self) -> None:
+        with pytest.raises(ValueError, match="AZURE_OPENAI_DEPLOYMENT"):
+            _isolated_settings(
+                database_url="postgresql://x:x@localhost/db",
+                llm_provider="azure_openai",
+                azure_openai_api_key="key",
+                azure_openai_endpoint="https://example.openai.azure.com",
+            )