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

.gitea/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test-backend:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - name: Install dependencies
+        run: bun install --frozen-lockfile
+
+      - name: Run tests
+        run: bun test --bail
+
+      - name: Type check
+        run: bun x tsc --noEmit