feat: add unit tests and Gitea Actions CI pipeline

- Extract pure utility functions to lib/utils.ts for testability
- Add 28 unit tests covering: computeNextDueDate, resetSubtasks,
  parseTaskIdentifier, validators, statusSortOrder
- Add Gitea Actions workflow (.gitea/workflows/ci.yml) that runs
  tests and type checking on push/PR to main
- Refactor tasks.ts to use extracted utils

backend/package.json

@@ -7,7 +7,9 @@
     "db:generate": "drizzle-kit generate",
     "db:migrate": "drizzle-kit migrate",
     "db:push": "drizzle-kit push",
-    "db:studio": "drizzle-kit studio"
+    "db:studio": "drizzle-kit studio",
+    "test": "bun test",
+    "test:ci": "bun test --bail"
   },
   "dependencies": {
     "@elysiajs/cors": "^1.2.0",