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

frontend/src/pages/ReplayPage.test.tsx

@@ -0,0 +1,85 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, screen, waitFor } from "@testing-library/react";
+import { MemoryRouter, Route, Routes } from "react-router-dom";
+import { ReplayPage } from "./ReplayPage";
+
+vi.mock("../api", () => ({
+  fetchReplay: vi.fn(),
+}));
+
+vi.mock("../components/ReplayTimeline", () => ({
+  ReplayTimeline: ({ steps }: { steps: unknown[] }) => (
+    <div data-testid="replay-timeline">{steps.length} steps</div>
+  ),
+}));
+
+import { fetchReplay } from "../api";
+const mockFetchReplay = vi.mocked(fetchReplay);
+
+beforeEach(() => {
+  mockFetchReplay.mockReset();
+});
+
+function renderWithRoute(threadId: string) {
+  return render(
+    <MemoryRouter initialEntries={[`/replay/${threadId}`]}>
+      <Routes>
+        <Route path="/replay/:threadId" element={<ReplayPage />} />
+      </Routes>
+    </MemoryRouter>
+  );
+}
+
+describe("ReplayPage", () => {
+  it("renders loading state initially", () => {
+    mockFetchReplay.mockReturnValue(new Promise(() => {}));
+    renderWithRoute("t1");
+    expect(document.querySelector(".skeleton-box")).toBeTruthy();
+  });
+
+  it("renders replay steps on success", async () => {
+    mockFetchReplay.mockResolvedValue({
+      thread_id: "t1",
+      total_steps: 2,
+      page: 1,
+      per_page: 100,
+      steps: [
+        { step: 1, type: "message", content: "Hello", agent: null, tool: null, params: null, result: null, timestamp: "2026-04-01T00:00:00Z" },
+        { step: 2, type: "response", content: "Hi!", agent: "bot", tool: null, params: null, result: null, timestamp: "2026-04-01T00:00:01Z" },
+      ],
+    });
+    renderWithRoute("t1");
+
+    await waitFor(() => {
+      expect(screen.getByTestId("replay-timeline")).toBeInTheDocument();
+    });
+    expect(screen.getByText("2 steps")).toBeInTheDocument();
+    // Thread ID appears in multiple places (header + sidebar)
+    expect(screen.getAllByText("t1").length).toBeGreaterThan(0);
+  });
+
+  it("renders empty state when no steps", async () => {
+    mockFetchReplay.mockResolvedValue({
+      thread_id: "t1",
+      total_steps: 0,
+      page: 1,
+      per_page: 100,
+      steps: [],
+    });
+    renderWithRoute("t1");
+
+    await waitFor(() => {
+      expect(screen.getByText("No replay steps found")).toBeInTheDocument();
+    });
+  });
+
+  it("renders error state on fetch failure", async () => {
+    mockFetchReplay.mockRejectedValue(new Error("Not found"));
+    renderWithRoute("t1");
+
+    await waitFor(() => {
+      expect(screen.getByText("Failed to load replay")).toBeInTheDocument();
+    });
+    expect(screen.getByText("Not found")).toBeInTheDocument();
+  });
+});