Move the auto-generated fallback chain definitions from
app/databus/provider_chains.py to app/databus/_generated/provider_chains.py
(underscore prefix = generated).
The legacy app/databus/provider_chains.py becomes an 8-line re-export shim.
Added scripts/generate_provider_chains.py to regenerate from the shim.
Phase 3B of AUDIT-2026-Q3.md.