fix(db): reject application models in migrations

[Guflly]

Fixes #623. Custom migrations now reject application models before they can access the database. Migration models and raw SQL keep working. Tested with the full cot suite, the macro suite, and clippy.

[codecov]

Codecov Report

❌ Patch coverage is 83.87097% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cot/src/db.rs	75.00%	0 Missing and 7 partials ⚠️
cot/src/db/migrations.rs	90.62%	1 Missing and 2 partials ⚠️

Flag	Coverage Δ
rust	90.28% <83.87%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cot-macros/src/model.rs	96.42% <100.00%> (+0.06%)	⬆️
cot/src/db/migrations.rs	83.87% <90.62%> (+0.40%)	⬆️
cot/src/db.rs	87.70% <75.00%> (-0.50%)	⬇️

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[Guflly]

Updated both. Database now uses DatabaseContext, and models expose a public non-exhaustive ModelType. The Cot and macro test suites and clippy pass.