Add Alembic database migration scripts for PostgreSQL

[Teboho66]

Description

When SentinelPay moves from in-memory storage to PostgreSQL, we need
database migration scripts to create the schema. This issue sets up
Alembic and creates the initial migration for the three core tables.

Tasks

• Install alembic and sqlalchemy
• Run alembic init migrations at repo root
• Create SQLAlchemy models for: transactions, fraud_cases,
  audit_records
• Generate initial migration: alembic revision --autogenerate
• Verify alembic upgrade head runs without errors against
  a local PostgreSQL instance

Acceptance Criteria

• alembic upgrade head creates all three tables
• alembic downgrade -1 rolls back cleanly
• Migration scripts are committed in migrations/versions/

Good first issue because

Alembic has excellent documentation. This task does not require
understanding the fraud detection logic.