Fix test job failure by deferring optional sentence_transformers loading

[Copilot]

The test GitHub Actions job failed during test collection because app.services.ingestion_service imported sentence_transformers at module import time. In CI environments where that package is unavailable, this prevented the test suite from starting.

• Root cause addressed: remove import-time hard dependency

  • Moved SentenceTransformer import from module scope into get_embedder().
  • Kept typing intact via TYPE_CHECKING so runtime import is deferred but signatures remain explicit.

• Startup resilience for optional embedder dependency

  • Updated app lifespan startup to catch ModuleNotFoundError around embedder warm-up.
  • Startup now continues with a warning when sentence_transformers is absent, instead of failing process initialization.

• Regression coverage for deferred import behavior

  • Added focused tests covering:
    • startup behavior when embedder import fails,
    • get_embedder() failure-path deferral semantics,
    • get_embedder() success-path initialization when sentence_transformers is available.

def get_embedder() -> "SentenceTransformer":
    global _embedder
    if _embedder is None:
        from sentence_transformers import SentenceTransformer
        _embedder = SentenceTransformer(settings.embedding_model)
    return _embedder