perf(retrieval): cherry-pick content trigram index to main

[suguanYang]

Summary

• cherry-pick the content trigram migration from staging into main
• preserve main branch migration contract tests while adding the content index contract

Source

• Staging PR: perf(retrieval): add content trigram index for regex grep #384
• Source commit: cc4690c

Verification

• uv run pytest apps/api/tests/migrations/test_schema_contract.py -q (16 passed)
• uv run ruff check (passed)
• git diff --check (passed)

This PR is intentionally based on main for the authorized cherry-pick delivery.