feat(db): support string concatenation in queries

[oxura]

Fixes #610.

I went with the explicit concat() shape we discussed in the issue rather than making Expr generic. It now supports all of these:

query!(Product, $name.concat("-") == "mortal-")
query!(Product, $name.concat($title) == "mortalkombat")
query!(Product, $name.concat(" ").concat($title) == "mortal kombat")

The query parser can now keep field references inside method arguments, Expr::Concat lowers to the portable CONCAT(lhs, rhs) function, and the direct field/value path is restricted to TextField receivers. Numeric + stays unchanged.

I added macro coverage for field/literal and field/field concatenation plus a real database test for chained concatenation. The SQLite scenario passes; the same test variants compile for PostgreSQL and MySQL and run when their test databases are available. The focused codegen and macro suites pass, as do clippy, formatting, and diff checks.

[codecov]

Codecov Report

❌ Patch coverage is 84.48276% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cot-codegen/src/expr.rs	76.19%	3 Missing and 2 partials ⚠️
cot-macros/src/query.rs	92.59%	2 Missing ⚠️
cot/src/db/query/expr.rs	80.00%	0 Missing and 2 partials ⚠️

Flag	Coverage Δ
rust	90.11% <84.48%> (-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/query.rs	91.62% <92.59%> (+0.04%)	⬆️
cot/src/db/query/expr.rs	74.31% <80.00%> (+0.32%)	⬆️
cot-codegen/src/expr.rs	84.00% <76.19%> (-0.18%)	⬇️

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