Generic single-database configuration for kryten-economy (PostgreSQL).

This directory is the single schema authority for the PostgreSQL backend
(Sprint 12 decision). There is no hand-rolled `sql/` directory and no custom
`schema_version` table: Alembic's `alembic_version` table tracks applied revisions.

`env.py` is schema-only. It does not import the asyncpg data layer, because the
economy store is written directly against asyncpg rather than a SQLAlchemy ORM.

Usage
-----
    # Apply migrations to the configured database
    uv run alembic upgrade head

    # Create a new revision after editing models/explicit DDL
    uv run alembic revision -m "describe the change"

    # Inspect SQL without connecting
    uv run alembic upgrade head --sql

The database URL is never stored in `alembic.ini` (it may contain a secret).
It is resolved at runtime from `KRYTEN_ECONOMY_ALEMBIC_URL`, or from the
service's `database.postgres` config block via `KRYTEN_ECONOMY_CONFIG`.
