Graceful degradation

Not every failure should fail the request. Decide, per dependency, whether its failure is fatal or degradable:

• Optional enhancement fails → degrade. If a document's preview rendering fails, keep the document usable for text and reading; don't reject the upload.
• Core dependency fails → fail clearly. If the model API is down, there's no answer to give; return an honest error.
• External enrichment fails → continue without it. If a research API is unavailable, answer from what you have and say what's missing, rather than crashing the turn.

Map each dependency to "fatal" or "degradable" deliberately, and wrap accordingly.