This PR adds user-policy associations and Alembic migration infrastructure.

User-Policy Associations

• Adds UserPolicy model to associate users with saved policies
• Adds CRUD endpoints at /user-policies/
• Adds 14 tests covering all endpoints

Endpoints

• POST /user-policies/ - create association
• GET /user-policies/?user_id=... - list by user (optional tax_benefit_model_id filter)
• GET /user-policies/{id} - get by id
• PATCH /user-policies/{id} - update label
• DELETE /user-policies/{id} - delete association

Alembic Migration Infrastructure

Added Alembic for database migrations:

• alembic.ini and alembic/ directory (env.py, script.py.mako)
• 0001_initial_schema - base tables (cherry-picked from Anthony's PR feat: Add household support (CRUD, associations, impact analysis) #77)
• 0002_add_user_policies - user_policies table + tax_benefit_model_id on policies

Archived old Supabase SQL migrations now handled by Alembic:

• 20251229000000_add_parameter_values_indexes.sql
• 20260103000000_add_poverty_inequality.sql
• 20260111000000_add_aggregate_status.sql

Note: When Anthony's PR #77 merges, we'll need an Alembic merge migration to reconcile both branches.