Test cache isolation: FastAPICache.init() no-ops, so the cache leaks between tests

Start with the shared fixture in src/quartz_api/internal/service/conftest.py, then inspect the FastAPICache.init calls and local fixture in src/quartz_api/internal/service/v1/test_router.py and test_status_router.py. Remove the redundant setup as described, run the full unit suite, and confirm the v1 cold-cache tests use isolated caches and all 223 tests pass.

FastAPICache.init() starts with if cls._init: return. The first call in a pytest session wins and every later call is a silent no-op.

Two consequences:

1. _create_server() re-inits the cache per test, but that does nothing after the first test — so the in-memory store is shared for the whole session and a cached route can serve a previous test's response. Hit this while adding coverage for GET /v0/solar/GB/status: the second test got the first one's payload. Existing tests hid it because their second case 404s, and exceptions aren't cached.

2. The ~25 FastAPICache.init(InMemoryBackend(), prefix="test") / prefix="cold" calls in src/quartz_api/internal/service/v1/test_router.py are all no-ops. They neither set the prefix nor provide a fresh backend. The cold-cache 503 tests are not actually starting from a cold cache — they pass on session ordering rather than on the isolation they look like they're asserting.

Fix: autouse fixture in src/quartz_api/internal/service/conftest.py clearing the backend between tests, then drop the no-op init calls in test_router.py.

@pytest_asyncio.fixture(autouse=True)
async def clear_cache():
    if FastAPICache._init:
        await FastAPICache.clear()
    yield

Verified: with this in the shared conftest the full unit suite passes (223 passed) with no exceptions needed — including the v1 cold-cache tests, which get a genuinely cold cache rather than an accidental one. FastAPICache.reset() is also available if a test wants a full re-init.

Interim: test_status_router.py carries a local copy of this fixture. Remove it as part of the fix.