feat: migrate test runner from Bun to Vitest

- Replace bun:test with Vitest across all 22 test files
- Create @sofa/test package with shared setup and DB test helpers
  - setup.ts: vi.mock for @sofa/db/client, Bun.randomUUIDv7 polyfill
  - db.ts: in-memory SQLite via better-sqlite3, seed helpers
- Add per-project vitest configs (packages/core, apps/web)
- Add root vitest.config.ts with projects and v8 coverage
- Set up @vitest/browser + Playwright for web component tests
- Move validateBackupDatabase from core/backup.ts to @sofa/db/client
- Derive required backup tables from schema instead of hard-coded list
- Update CI workflow with Playwright install and Codecov upload
- Update CLAUDE.md documentation for Vitest

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/test/image-cache.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, beforeEach, describe, expect, spyOn, test } from "bun:test";
+import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
 
 import { loadImageBuffer } from "../src/image-cache";
 
@@ -13,7 +13,7 @@ afterEach(() => {
 describe("loadImageBuffer", () => {
   test("uses category-specific TMDB sizes when cache is disabled", async () => {
     const urls: string[] = [];
-    const fetchSpy = spyOn(globalThis, "fetch").mockImplementation((async (
+    const fetchSpy = vi.spyOn(globalThis, "fetch").mockImplementation((async (
       input: string | URL | Request,
     ) => {
       urls.push(String(input));