Refuse bulk backfill on a non-Anthropic default instead of no-op

Refuse bulk backfill on a non-Anthropic default instead of no-op POST /ai/backfill submits one provider Batch job, and only Anthropic offers that API. With an openai_compatible default the handler queued a job the worker then silently skipped ('backfill submitted nothing'), so the caller saw a job id and outstanding count for work that never ran. Now the handler checks the default credential's provider up front and, for anything but Anthropic, returns 422 pointing at per-asset enrich; with no active credential it says so too. Adds an ai_api regression test.