fix(webapp): prevent duplicate envs from provision race

[carderne]

Fixes TRI-12078

Summary

Prevents concurrent environment setup requests from creating duplicate Staging and Preview environments.

Fix

Adds database-enforced uniqueness for root Staging and Preview environments.

If two requests race, the losing request loads the environment created by the winner and continues successfully instead of creating a duplicate or returning an error.

[changeset-bot]

⚠️ No Changeset found

Latest commit: db6bd6a

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[coderabbitai]

Walkthrough

Adds a partial unique index enforcing one root Staging or Preview environment per project and runtime type. Updates environment upsert logic to identify root environments, handle Prisma unique-constraint conflicts from overlapping creation requests, re-query the existing environment, and update its concurrency limits. Adds schema documentation and a server-changes entry.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description covers the fix but misses most required template sections, including checklist, testing, changelog, and screenshots.	Add the missing template sections: Closes #, checklist, testing steps, changelog, and screenshots placeholder.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title is concise and accurately reflects the main fix: preventing duplicate environments during a provisioning race.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/duplicate-root-environments-tri-12078

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[carderne]

Do not merge until this has been backfilled.

[devin-ai-integration]

Devin Review found 0 potential issues.