refactor: move service/llm_keys.py out of the service layer (CODE_HEALTH #4)

[duanyiqun]

CODE_HEALTH #4 (see docs/CODE_HEALTH.md in #37).

cognifold.service.llm_keys is imported by five packages below the service layer: agent/ (7 sites), query/ (incl. query/llm.py:16), utils/, embeddings/, and config.py:161 — meaning the root config module depends on the HTTP service layer, inverting the documented layering (docs/ARCHITECTURE.md §Module Dependencies). A visible symptom: 287 function-local from cognifold... imports across src/ exist to dodge the resulting cycles.

llm_keys.py has no FastAPI dependency — it is simply misfiled.

Fix direction (small + mechanical, good first refactor):

• Move service/llm_keys.py → cognifold/llm/keys.py (or utils/)
• Keep a thin deprecation re-export at the old path
• Update the ~13 import sites; promote now-safe deferred imports to top level
• Add a note/lint guard so nothing below service/ imports from it again (rule already stated in CLAUDE.md)