[codex] support text direct downloads

[monlor]

Summary

• return text shares as UTF-8 plain-text attachments from GET /share/select/
• use the share name with a .txt fallback for the downloaded filename
• preserve the existing expiration, download-count, and usage-accounting behavior
• cover direct text downloads with a regression test

Why

The direct-download endpoint always delegated to the configured file storage. Text shares do not have a physical file, so valid text shares were incorrectly reported as deleted or expired when accessed through a direct link.

Impact

Direct links now work for both uploaded files and text shares. Existing file-download behavior is unchanged.

Validation

• .venv/bin/python -m unittest tests.test_issue_482_share_usage -v
• .venv/bin/python -m compileall -q apps/base/views.py tests/test_issue_482_share_usage.py
• git diff --check
• full test run: 31 tests passed; 2 unrelated theme-asset tests fail because themes/2023 assets are absent from this checkout

[vercel]

@monlor is attempting to deploy a commit to the vastsa's projects Team on Vercel.

A member of the Team first needs to authorize it.