test: pytest suite (31 tests) + lazy config validation + CI pytest job

- src/server.py: CALDAV_DATA_PATH validation moved from import-time to
  _validate_startup_config(), called on __main__ or MCP_ICS_VALIDATE_ON_IMPORT=1.
  Module is now importable in tests without env setup.
- tests/: Radicale fixture tree (VCALENDAR/VADDRESSBOOK, hidden dirs, corrupt
  props, bad ics), covers discover_calendars, parse_ics_files, component_to_dt,
  format_dt, format_component (exclude/birthday filters, VEVENT vs VTODO shape),
  get_events_in_range (RRULE expansion, cross-calendar aggregation, sort),
  get_agenda (default +7d end, error dict, flag forwarding), /health endpoint,
  _validate_startup_config. 31 tests, <2s.
- .gitea/workflows/_ci-common.yaml: new pytest job running the test suite
  inside the built CI image (prod requirements).
- pkg/Dockerfile: COPY tests/

.gitea/workflows/_ci-common.yaml

@@ -32,3 +32,25 @@ jobs:
           cache-to: type=gha,mode=max
       - name: Smoke test (compile sanity)
         run: docker run --rm --name mcp-ics-smoke-$(hostname) ci-image:${{ github.sha }} python -m compileall -q src/
+
+  pytest:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
+      - name: Build (cached)
+        uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
+        with:
+          context: .
+          file: pkg/Dockerfile
+          push: false
+          load: true
+          tags: ci-image:${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+      - name: Unit tests (pytest, in-image env with prod requirements)
+        run: >
+          docker run --rm ci-image:${{ github.sha }}
+          sh -c "pip install pytest --quiet && python -m pytest tests/ -q"