Normalize v2 document storage and human views

[codeforester]

Summary

• Add v2 financial storage roots plus document object/view metadata tables under the BB_ schema.
• Add storage DAO/path helpers for relative key validation, path resolution, deterministic object keys, and read-only managed-file guardrails.
• Expose the separate financial/ directory namespace and update architecture docs/open questions around canonical storage and generated human views.
• Drop the superseded v2 path columns from BB_DOCUMENT and BB_IMPORT_ATTEMPT so future v2 code uses object/view references.

Validation

• uv run pytest tests/test_financial_storage.py tests/test_financial_foundation.py tests/test_database.py -q
• uv run pytest -q
• ./tests/validate.sh
• git diff --check
• git diff --cached --check

Closes #127