fix: drop unsupported .csv from self-hosted env examples

[Frankie-Xu]

Related to self-hosted Quick Start (cp apps/api/.env.example apps/api/.env and the worker equivalent).

Summary

• Remove .csv from SUPPORTED_EXTENSIONS in apps/api/.env.example and apps/worker/.env.example.
• Worker format_router.SUPPORTED_FILE_TYPES / resolve_document_format() cannot parse .csv (no CSV adapter; it raises ValidationException).
• storage.py SUPPORTED_EXTENSIONS default also omits .csv.
• README supported formats do not list .csv.
• Example files advertised .csv, so a self-hosted operator following README Quick Start could pass API admission and then hard-fail in the worker.

Example-only. No production default, admission behavior, parser, or README change.

Not in this PR (admission-contract gap, needs maintainer decision)

Worker format_router and README list .json as supported, but storage.py's SUPPORTED_EXTENSIONS default omits .json. Out-of-the-box (no env override) the API rejects a format the worker can parse. Changing that default is a public admission-behavior change, so it is not included here. Tracked as #418.

Verification

• make lint — All checks passed
• make typecheck — 0 errors, 0 warnings, 0 informations
• Confirmed SUPPORTED_FILE_TYPES still has no .csv
• Confirmed storage.py default still omits .csv
• Confirmed README supported list still omits .csv

Deployment Notes

• No new environment variables. Existing deployments that copied the old example and kept .csv are unchanged until they recopy the example.
• No database migrations, queue changes, storage changes, or release-order requirements
• Backwards compatible for production defaults; rollback is reverting this PR

Checklist

• Tests were added or updated when behavior changed — N/A (example-only)
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — none; production SUPPORTED_EXTENSIONS default unchanged