Add nightly dry-run workflow

.forgejo/workflows/nightly.yml

@@ -0,0 +1,33 @@
+name: Nightly dry-run
+
+on:
+  schedule:
+    - cron: '0 2 * * *'
+
+jobs:
+  dry-run:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install dependencies
+        run: pip install -r requirements.txt
+
+      - name: Dry-run pipeline
+        run: |
+          python3 -c "
+          import sys
+          sys.path.insert(0, 'src')
+          from transform.clean import drop_nulls, deduplicate
+          import pandas as pd
+          df = pd.DataFrame({'id': [1, 1, 2, None], 'val': ['a', 'a', 'b', 'c']})
+          df = drop_nulls(df, ['id'])
+          df = deduplicate(df, ['id'])
+          assert len(df) == 2, f'Expected 2 rows, got {len(df)}'
+          print('Dry-run OK')
+          "