feat(tests): fixture sweep, anonymized real-invoice regression corpus, CI

- test_fixture_sweep: extract+validate every fixture PDF on every run;
  known extractor/validator gaps are catalogued explicitly and the sweep
  fails on any deviation from that catalogue
- tests/fixtures/real/: anonymized XML from real production invoices that
  revealed bugs (verified to contain no real-world data); first entry
  locks in the FC-tax-number/basis-quantity/spaced-VAT-ID regressions
- CI workflow: uv sync --frozen from lockfile + pytest + docker build

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: CI
+
+on:
+  push:
+    branches: [master, main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
+        with:
+          enable-cache: true
+
+      - name: Install dependencies from lockfile
+        run: uv sync --extra dev --frozen
+
+      - name: Run tests
+        run: uv run --extra dev pytest -q
+
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Docker image
+        run: docker build -t zugferd-service:ci .