🛢️ Add Alembic for Database Migrations

This PR introduces Alembic to manage schema migrations for the mcpgateway database to support #151 and #176 - as well as future database schema changes and migrations. See ./alembic/README.md in this PR.

✅ Highlights

• Added Alembic scaffolding (alembic.ini, alembic/env.py, script.py.mako, versions/)
• Integrated with existing SQLAlchemy models (Base.metadata)
• Supports autogeneration based on model diffs
• Included Makefile targets for common migration tasks

🧪 Usage

Create a migration:

make db-new MSG="add users table"

Apply migrations:

make db-up

Rollback one revision:

make db-down

Inspect state:

make db-current      # Show applied revision
make db-history      # Show full revision tree

🧠 Notes

• Migrations are stored in alembic/versions/.
• The DB URL is read dynamically from settings.database_url.
• All models must be properly imported for autogeneration to detect changes.