Replace non-atomic db.executescript() with explicit transaction control.
Wrap each migration's DDL statements and schema_migrations insert in a
single BEGIN IMMEDIATE ... COMMIT transaction to ensure atomicity.
Changes:
- Add _parse_migration_statements() to split migration scripts into
individual statements while handling comments and string literals
- Update _apply_migration() to wrap all statements in a single explicit
transaction with rollback on error
- Ensure _get_current_schema_version() uses execute() instead of
executescript()
- Add 9 new tests for migration atomicity and statement parsing
- Update Backend-Development.md with migration authoring guidelines
If a crash occurs between DDL execution and schema_migrations insert,
the next startup will re-apply the entire migration atomically,
preventing partial migrations and data corruption.
Test coverage: 98% on db.py (up from 55%)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
