feat(backend): add /diagnostics/flags + /diagnostics/telemetry routes

- GET /api/diagnostics/flags — returns all feature flags
- GET /api/diagnostics/telemetry — returns buffered telemetry events
- POST /api/diagnostics/telemetry/flush — flush telemetry buffer
- Fix server.test.ts mock to include get/post + feature-flags/telemetry mocks
- 80/80 tests pass

backend/src/server.test.ts

@@ -8,6 +8,8 @@ const initDatastoreMock = vi.fn(() => undefined);
 
 const appMock = {
   register: vi.fn(async () => undefined),
+  get: vi.fn(),
+  post: vi.fn(),
 };
 
 vi.mock('@bytelyst/fastify-core', () => ({
@@ -39,6 +41,8 @@ vi.mock('./lib/config.js', () => ({
   },
 }));
 vi.mock('./lib/product-config.js', () => ({ DISPLAY_NAME: 'NoteLett' }));
+vi.mock('./lib/feature-flags.js', () => ({ getAllFlags: vi.fn(() => ({})) }));
+vi.mock('./lib/telemetry.js', () => ({ getBufferedEvents: vi.fn(() => []), flushEvents: vi.fn(() => []) }));
 
 describe('server bootstrap', () => {
   beforeEach(() => {

backend/src/server.ts

@@ -10,6 +10,8 @@ import { workspaceRoutes } from './modules/workspaces/routes.js';
 import { initCosmosIfNeeded } from './lib/cosmos-init.js';
 import { initDatastore } from './lib/datastore.js';
 import { config } from './lib/config.js';
+import { getAllFlags } from './lib/feature-flags.js';
+import { getBufferedEvents, flushEvents } from './lib/telemetry.js';
 import { DISPLAY_NAME } from './lib/product-config.js';
 import type { JwtPayload } from './lib/request-context.js';
 
@@ -53,4 +55,9 @@ await registerApiPlugin(noteTaskRoutes);
 await registerApiPlugin(savedViewRoutes);
 await registerApiPlugin(workspaceRoutes);
 
+// ── Diagnostics routes (no auth) ────────────────────────────────
+app.get('/api/diagnostics/flags', async () => getAllFlags());
+app.get('/api/diagnostics/telemetry', async () => ({ events: getBufferedEvents() }));
+app.post('/api/diagnostics/telemetry/flush', async () => ({ flushed: flushEvents().length }));
+
 await startService(app, { port: config.PORT, host: config.HOST });