test(coverage): pytest-cov einbauen (term-missing Report)

Kein --cov-fail-under noch gesetzt, erst Baseline auf 137 ermitteln.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Ahyx6D3r7G1EuAc42nezn

backend/pytest.ini

@@ -5,3 +5,4 @@ asyncio_default_test_loop_scope = session
 testpaths = tests
 python_files = test_*.py
 python_functions = test_*
+addopts = --cov=app --cov-report=term-missing

backend/requirements.txt

@@ -22,6 +22,7 @@ icalendar==7.0.3
 pytest==9.0.2
 pytest-asyncio==1.3.0
 pytest-httpx==0.36.0
+pytest-cov==7.0.0
 aiosqlite==0.22.1
 weasyprint==68.1
 typer==0.25.1