Implement proper schema migrations for ALTER TABLE

All tests pass. The implementation adds:

1. **`runMigrations`** - Called at the end of `initTaskDb` to run
migrat 2. **`migrateTable`** - Compares expected columns against
existing colum 3. **`getTableColumns`** - Uses `PRAGMA table_info` to
get existing colu 4. **`addColumn`** - Runs `ALTER TABLE ADD COLUMN`
for missing columns 5. **Column definitions** - Lists of expected
columns for `task_activity

When new columns are added to the schema in the future, you just
add the

Task-Id: t-152.3

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