Lukas a44f1ef35b TASK-023: Make database migrations atomic ...

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>

2026-04-26 14:40:27 +02:00

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