Migrate API v1 persistence to SQLAlchemy and Alembic

[anth-volk]

Fixes #3787

Summary

• Migrates API v1 persistence to service-owned SQLAlchemy 2 Sessions and direct mapped models.
• Keeps routes and legacy endpoints limited to validation, service calls, and response serialization; they no longer construct or pass Sessions.
• Removes the DAO, repository, unit-of-work, raw database facade, and session-manager compatibility layers.
• Makes simulation and report multi-model writes atomic and returns immutable result objects for orchestration workflows.
• Keeps database sessions closed during household calculation, simulation-gateway calls, and Claude streaming.
• Retains the autogenerated Alembic baseline, read-only schema comparison, and model-agnostic migration guidance.
• Documents local SQLite bootstrapping as temporary infrastructure scheduled for replacement by a shared cache in Stage 8.

Canonical SQLAlchemy boundary

• One lazy Engine and pool are owned per worker process.
• The Cloud SQL connector supplies DBAPI connections through SQLAlchemy's creator hook; SQLAlchemy owns pooling and connection lifecycle.
• Application services own short-lived read Sessions and sessionmaker.begin() write transactions.
• Services query and mutate mapped models directly; internal transaction-scoped helpers may accept a Session.
• Dependencies are lazily defaulted in production and explicitly injectable in tests.
• Application startup performs no MySQL DDL, and shutdown disposes Engines and closes Cloud SQL connectors.

Validation

• Complete migration regression gate: 997 passed (tests/to_refactor, tests/unit, tests/contract, and the budget-window integration test).
• Architecture tests enforce persistence-free routes/endpoints, persistence-free public service signatures, and continued removal of compatibility abstractions.
• Transaction tests cover commit, rollback, generated IDs, atomic multi-model writes, and closed sessions during external or compute work.
• Changed Python modules pass Ruff and formatting checks; git diff --check passes.
• No schema-model or Alembic revision change was required by the service-owned Session conversion.
• The existing optional live-schema comparison remains read-only and requires an explicit STAGE7_EXISTING_DATABASE_URL.