Models, sessions, and Alembic migrations

A schema changes more often than the code that reads it stays still. Adding a customers table, a foreign key from tickets, a priority column next month: each change has to reach every database the app runs against, in order, exactly once. Alembic is the tool that makes a schema change a file you can review, run, and roll back, and the async template it ships with fits the engine from the last lesson without adaptation.

Before migrations, though, two smaller things need to be right: how models relate to each other, and how a session gets into a request handler and out again.