[$25 BOUNTY] [Python] Add migration dry-run planning tests

Description

**Bounty:** $25 (LT) **Area:** tools/db_migration.py **Current state:** The migration CLI has a dry-run path but the status/apply flow does not provide a machine-readable plan that can be asserted in tests before touching a database. **What's needed:** Add a deterministic dry-run planning mode for pending migrations and tests that verify the generated plan without requiring PostgreSQL. **Acceptance criteria:** - `tools/db_migration.py` can produce a dry-run migration plan without executing `psql`. - The plan includes migration version, description, direction, and whether execution would be attempted. - Tests cover no pending migrations, pending migrations, and a specific rollback target. - Existing `--up`, `--down`, and `--status` behavior remains backward compatible. **Required validation:** - Run `python3 build.py` - Include the generated diagnostic `.logd` artifact from `diagnostic/build-XXX.logd` in the PR; also include `diagnostic/build-XXX.json` if present - Use `.github/pull_request_template.md` for your submission