Fix MinerU upload mode flag

[suguanYang]

Summary

Closes #83.

• Add MINERU_UPLOAD_MODE_ENABLED as the explicit MinerU direct-upload mode flag, defaulting to false.
• Stop using ENVIRONMENT to decide MinerU URL mode; when upload mode is disabled and a reusable s3_key exists, the worker uses S3 URL mode.
• Drop the old FORCE_MINERU_UPLOAD_ENABLED configuration path instead of maintaining a legacy alias.
• Document that LocalStack/private storage URLs should set MINERU_UPLOAD_MODE_ENABLED=true to force direct upload.
• Leave MinerU timeout behavior unchanged.

Verification

• uv run pytest apps/worker/tests/contract -q
• uv run ruff check
• uv run pyright
• git diff --check
• git diff --cached --check