Fix data dir for Vercel serverless

[gitgrahamdunn]

Motivation

• Import-time creation of .data under the repo caused OSError: [Errno 30] Read-only file system on Vercel because /var/task is read-only, so the data directory must be moved to a writable location.

Description

• Change DEFAULT_DATA_DIR resolution to prefer the DATA_DIR environment variable and otherwise fall back to /tmp/gitplant-data so serverless functions can write to a writable path.
• If a configured or computed candidate directory is inside the repository (REPO_ROOT), the code falls back to /tmp/gitplant-data to avoid writes under /var/task.
• Keep idempotent directory creation with DEFAULT_DATA_DIR.mkdir(parents=True, exist_ok=True) so existing directories are fine and no extra behavior is changed.
• Add a short comment that Vercel serverless functions run on a read-only filesystem except for /tmp to explain the rationale.

Testing

• Ran python -m py_compile backend/app/config.py and it completed successfully.
• No other automated tests were modified or run as part of this change.

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Codex Task

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
gitplant	Ready	Preview, Comment	Feb 18, 2026 10:01pm
gitplant-backend	Ready	Preview, Comment	Feb 18, 2026 10:01pm
gitplant-oggy	Ready	Preview, Comment	Feb 18, 2026 10:01pm

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Normalize DATA_DIR before enforcing repo-root fallback

The repo-root guard uses candidate.relative_to(REPO_ROOT) on the raw path, so DATA_DIR values containing traversal segments (for example /tmp/../var/task/.data or ../var/task/.data) are treated as outside the repo even when they resolve to /var/task/.... In that case _default_data_dir() returns a path under the read-only Vercel tree and DEFAULT_DATA_DIR.mkdir(...) can still fail at import time, which defeats the fix this commit is trying to provide. Resolve/canonicalize candidate before the relative_to check so equivalent paths inside REPO_ROOT are consistently redirected to /tmp.

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