Refactor schema profiles and resolution

[ileodo]

Summary

• Move schema field definitions, raw row handling, and database-specific resolution into the new schema package.
• Add SchemaProfile so model fields are resolved through inherited field definitions, with errors grouped by field. MoneywizApi can accept a custom profile and validates it before loading.
• Resolve tag join table metadata from the database and pass the resolved profile through the accessor and managers.
• Create records through the profile, including plain dataclass subclasses, and document custom profile usage in the README.
• Let profile validation discover loaded Record subclasses by default while accepting an explicit model list for scoped validation.

Test evidence

• ruff format src tests — applied.
• ruff check src tests — passed.
• pytest -q — 26 passed, 15 skipped. The skipped integration tests require MONEYWIZ_TEST_DB_PATH.
• git diff --check — passed.