Refactor the storage layer to use SQLAlchemy with a PostgreSQL backend.

Proposed changes:
- Create a DecisionModel class using SQLAlchemy's declarative base
- Store decisions in a decisions table with columns: id, decision, rationale, scope, constraints
- Add an Alembic migration for the initial schema
- Replace MemoryStore.load() with a SQLAlchemy session using psycopg2

This gives us type safety, query flexibility, and a proper migration layer for
future schema evolution.
