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/

pkg/Dockerfile

@@ -16,5 +16,8 @@ RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
 # Copy the source code
 COPY src/ ./src/
 
+# Copy tests (used by CI; harmless in production)
+COPY tests/ ./tests/
+
 # Command to run the MCP server
 CMD ["python", "src/server.py"]