Add comprehensive backup/import unit tests and extract shared test helpers

Extract shared test utilities (createTestDatabase, seedUserData, loadSnapshot,
etc.) from d1-api.test.ts into tests/test-helpers.ts. Add 18 new tests in
tests/backup.test.ts covering YAML round-trip fidelity, validation/error
handling, database round-trip, and case-insensitive matching. Also fix missing
is_skipped column in the test schema.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: garyo

package.json

@@ -7,7 +7,7 @@
     "dev": "npm run version:generate:dev && npm run sw:generate && astro dev",
     "build": "npm run validate:data && npm run version:generate && npm run sw:generate && astro build",
     "preview": "astro preview",
-    "test": "node --import tsx --test tests/d1-api.test.ts",
+    "test": "node --import tsx --test tests/d1-api.test.ts tests/backup.test.ts",
     "astro": "astro",
     "version:generate": "node scripts/generate-version.js",
     "version:generate:dev": "NODE_ENV=development node scripts/generate-version.js",