test: add analysisOnly regression tests; fix cache isolation in tests

New file tests/experimentDailyStatuses.test.js — 9 tests covering
GET /api/experiments/:id/daily-statuses:
  - 404 on unknown experiment
  - all statuses returned when no filter
  - analysisOnly=true keeps only non-null analysis_summary records
  - analysisOnly=true returns [] when none have analysis_summary
  - the exact null/object mix that caused the production 500 now passes
  - analysisOnly=false (explicit) returns all records
  - ?date= param is forwarded to Prisma correctly
  - empty-array response on zero statuses
  - returned records include expected fields

Fixed tests/calendar.test.js: existing tests were silently broken by the
cache added in the perf commit. Both test files now mock lib/cache to
always miss, so each test exercises the real route logic in isolation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/calendar.test.js

@@ -1,6 +1,8 @@
 const request = require('supertest');
 
 jest.mock('../src/lib/prisma', () => require('./__mocks__/prisma'));
+jest.mock('../src/lib/cache', () => ({ get: jest.fn(() => undefined), set: jest.fn(), del: jest.fn(), delByPrefix: jest.fn() }));
+
 const prisma = require('../src/lib/prisma');
 const app = require('../src/app');