perf(webapp): read per-run environment config from the replica at dequeue

[ericallam]

Summary

Adds an opt-in path to serve a run's per-run configuration reads from the control-plane read replica instead of the primary, reducing primary database load during task execution. The managed-worker dequeue resolves each run's environment, organization, and environment variables before starting the run; those rows are stable for the life of a run, so they can safely come from the replica.

Gated by CONTROL_PLANE_DEQUEUE_READS_FROM_REPLICA, defaulting to "0" (reads from the primary, unchanged from today). Set it to "1" to route the reads to the replica. The env-var read is scoped to the dequeue/resolution path (resolveVariablesForEnvironment); dashboard env-var reads and writes always stay on the primary. When no read replica is configured, $replica transparently falls back to the writer, so single-database self-host is unchanged either way.

Verified end-to-end against a real primary/replica split, in both trigger dev and deployed (managed-worker) runs: with the flag on, env vars inject correctly and a value set immediately before triggering a deployed run is present on the run.

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Latest commit: c3521aa

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[coderabbitai]

Walkthrough

Environment variable retrieval now accepts optional parent-environment and replica-read options. Secret loading selects the replica Prisma client when requested, and environment resolution requests replica reads. Worker runtime-environment lookups select the replica or primary client based on CONTROL_PLANE_DEQUEUE_READS_FROM_REPLICA. The environment schema defines this setting with a default of "0". Integration coverage verifies replica and writer routing.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 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 clearly identifies the performance change and the replica-read path for per-run environment configuration.
Description check	✅ Passed	The description clearly explains the change, configuration flag, fallback behavior, scope, and testing, but omits the template checklist and changelog sections.

✨ 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 feat/cp-dequeue-reads-to-replica

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[devin-ai-integration]

Devin Review found 3 potential issues.