API config.py now reads CARTSNITCH_DATABASE_URL first, falls back to
DATABASE_URL (which the infra K8s overlay sets for all pods), and finally
falls back to the hardcoded default. Also normalizes plain postgresql://
to postgresql+asyncpg:// for the asyncpg driver.
Fixes CAR-510.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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