feat(webapp): make the default realtime backend configurable

[ericallam]

Summary

The default realtime backend was hardcoded to Electric. This adds a REALTIME_BACKEND_DEFAULT env var (electric | native | shadow, default electric) that chooses the backend for any environment whose org has no realtimeBackend override. Behavior is unchanged unless you set it; per-org overrides still win.

The default is applied at every point where the per-org flag falls through: the initial value, the flag lookup default, and the error fallback.

[changeset-bot]

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Latest commit: a0d2a4d

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[coderabbitai]

Walkthrough

Added the validated REALTIME_BACKEND_DEFAULT environment variable, supporting electric, native, and shadow, with electric as the default. Updated realtime backend resolution to use this value for initialization, feature-flag defaults, and error fallbacks. Documented the new environment variable and its behavior.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description only includes a summary and omits required template sections like Closes #issue, checklist, testing, changelog, and screenshots.	Add the missing template sections: Closes #issue, checklist items, Testing steps, Changelog, and Screenshots or explicit N/A entries.

✅ 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 clearly states the main change: making the default realtime backend configurable.

✨ 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 feature/tri-11906-add-realtime_backend_default-env-var-to-set-the-default

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.