fix: inject tenant scope into ORM queries

[yaojin3616]

Summary

Moves tenant filtering from individual DAO and business queries into SQLAlchemy's ORM session execution layer.

Root cause

Tenant isolation depended on each API, service, and DAO remembering to add a tenant_id predicate. More than a thousand direct API/service ORM queries bypass the DAO layer, so any missed predicate could expose another tenant's records.

Changes

• Inject the active ContextVar tenant predicate for tenant-owned ORM SELECTs, including direct API/service queries.
• Treat models with non-null tenant_id columns as tenant-owned by default and explicitly mark legacy nullable tenant-owned models.
• Add a regression test proving a direct ORM query without a tenant filter returns only the active tenant's rows.

Validation

• PYTHONPATH=. uv run pytest tests/test_base_dao.py
• PYTHONPATH=. uv run ruff check app/dao/base.py tests/test_base_dao.py
• bash scripts/arch-guard.sh