Add ID-strategy guidance: int vs UUID, autoincrement, client-set IDs

[developerjamiu]

Demand

Kapa "User ID & UUID Management" cluster — 9 conversations / 6 users.

Problem

06-database/02-models.md covers id type customization, but does not:

• Give int-vs-UUID decision guidance (performance / storage / use case).
• State plainly that serial means auto-increment, or explain serverOnly + autoincrement.
• Cover setting a UUID from the client (and the security implications).
• Show having both a UUID id and a secondary serial int on one model.

Scope

• A "Choosing an ID strategy" section in 06-database/02-models.md covering the above, with an example model carrying both a UUID id and a serial int.