fix(backend): resolve pytest/FastAPI/Pydantic deprecation warnings and fix shallow-copy mutation bug

[littlecoffe8]

Description

Resolves every warning currently emitted by the test suite (7 → 0). One of them was masking a real bug, fixed here rather than silenced.

Code changes

• app/business/open_food_facts/panel_renderer/generator.py (bug fix, not just a warning fix): apr.copy() → apr.model_copy(deep=True). The old shallow copy left mock.breeding_type_and_quantity pointing at the same nested object as the original apr. Rendering a "missing quantity" panel then mutated the placeholder quantity (EggQuantity.from_count(1)) onto that shared object, silently corrupting the original AnimalPainReport it was supposed to leave untouched. Added test_knowledge_panel_generator_missing_quantity_does_not_mutate_original_pain_report as a regression test (verified it fails without the fix).
• app/main.py: replaced the deprecated @app.on_event("shutdown") with a lifespan context manager, per FastAPI's current guidance.
• pyproject.toml: set asyncio_default_fixture_loop_scope = "function" to silence pytest-asyncio's "unset fixture loop scope" deprecation warning (and pin the intended, current default behavior explicitly rather than relying on an implicit one that's changing).
• tests/app/api/open_food_facts/test_routes.py: 3 pre-existing tests set mock_response.raise_for_status = AsyncMock(...), but raise_for_status() is called synchronously (never awaited) in the real code - calling that mock produced an un-awaited-coroutine RuntimeWarning. Changed to Mock(...), consistent with the other ~10 tests in the same file that already do this correctly.

How to test

cd backend
uv sync --all-groups
uv run pybabel compile -d app/locales
uv run pytest tests/

Before: 110 passed, 7 warnings. After: 111 passed (no warnings line at all).