Adds schema_versions table and migrations.py runner to prevent
double-application and missed migrations across dev/prod environments.
- migrations/2026-03-27_add_schema_versions_table.sql: creates tracking table
- migrations.py: applies pending .sql files in sorted order, records each in schema_versions
- .gitignore: untrack migrations.py (was incorrectly ignored as legacy root file)
First run on an existing DB will apply all migrations (safe — all use IF NOT EXISTS).
Closes#81
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>