perf: index owner-scoped email timelines

[seonghobae]

Summary

• add ix_email_records_owner_date on (user_id, organization_id, date)
• keep SQLAlchemy model metadata and bootstrap backfill SQL aligned
• add bootstrap/model regression coverage for the composite index

Supersedes the useful database-performance slice of #574. The older #574 branch is based on master and also carries stale Python/dependency/workflow churn that is already present or newer on develop.

Verification

• python3 -m pytest backend/tests/test_bootstrap_db.py -q (22 passed, 1 skipped; pytest emitted pre-existing warning text from the current harness)

Security Notes

• No new input, URL, subprocess, IPC, WebView, model, or export boundary is introduced.
• The change is a database index only; tenant/user scoping semantics remain unchanged.
• Bootstrap SQL uses CREATE INDEX IF NOT EXISTS for safe repeat execution.

[seonghobae]

Current status for head 50217d38d0842b2a1fe47bf3063833272c31d8f9:

• Reviewed the PR perf: index owner-scoped email timelines #591 diff for the owner-scoped email timeline index. The change is limited to the SQLAlchemy model index, bootstrap backfill SQL, regression tests, and .jules/bolt.md guidance.
• GitHub GraphQL reports unresolved_threads=0, reviewDecision=null, and mergeStateStatus=BLOCKED.
• Local verification passed on the PR branch:
  • python3 -m pytest backend/tests/test_bootstrap_db.py -q (22 passed, 1 skipped)
  • python3 -m ruff check backend/db/models.py backend/scripts/bootstrap_db.py backend/tests/test_bootstrap_db.py
  • git diff --check origin/develop...HEAD
• I intentionally did not include black --check as PR evidence because the same command fails on untouched origin/develop for these files; formatting drift is pre-existing, not introduced by this PR.
• Current-head GitHub Actions jobs do not start runner steps. Check-run annotations report: The job was not started because your account is locked due to a billing issue.

[github-actions]

PR governance metadata gate is not ready for 65ee0eb5043c94618f82d67f64cd22a4a04b9896:

• Merge state is UNKNOWN; resolve conflicts or refresh mergeability.
• Required check metadata could not be read: no required checks reported on the 'fix/email-owner-date-index-20260619' branch.