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 84.93151% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cot/src/db.rs	78.78%	0 Missing and 7 partials ⚠️
cot/src/db/migrations.rs	88.57%	1 Missing and 3 partials ⚠️

Flag	Coverage Δ
rust	90.42% <84.93%> (-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	96.52% <100.00%> (+0.15%)	⬆️
cot/src/db/migrations.rs	83.80% <88.57%> (+0.33%)	⬆️
cot/src/db.rs	88.39% <78.78%> (-0.44%)	⬇️

... and 1 file with indirect coverage changes

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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.

[seqre]

Thank you for your contribution and from my side it seems fine! I'll leave the final call to @m4tx

[m4tx]

Looks good to me, thanks a lot for this!