Set up Mage build targets

Add magefiles/magefile.go with build automation targets:
- Test() - runs go test -v ./...
- Coverage() - runs tests with coverage profile output
- Lint() - runs staticcheck for static analysis
- Fmt() - formats all Go source files with gofmt
- Vet() - runs go vet for code analysis
- Check() - runs all quality checks in sequence (fmt, vet, lint, test)
- Clean() - removes build artifacts

Update magefiles/go.mod with github.com/magefile/mage dependency.

Closes checkvist-api-8q3

magefiles/go.mod

@@ -1,3 +1,5 @@
 module code.beautifulmachines.dev/jakoubek/checkvist-api/magefiles
 
 go 1.21
+
+require github.com/magefile/mage v1.15.0

magefiles/go.sum

@@ -0,0 +1,2 @@
+github.com/magefile/mage v1.15.0 h1:BvGheCMAsG3bWUDbZ8AyXXpCNwU9u5CB6sM+HNb9HYg=
+github.com/magefile/mage v1.15.0/go.mod h1:z5UZb/iS3GoOSn0JgWuiw7dxlurVYTu+/jHXqQg881A=

magefiles/magefile.go

@@ -3,4 +3,61 @@
 // Package main provides Mage build targets for the checkvist-api module.
 package main
 
+import (
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/magefile/mage/mg"
+	"github.com/magefile/mage/sh"
+)
+
 // magefile.go contains build targets: Test, Coverage, Lint, Fmt, Check.
+
+// Default target when running mage without arguments.
+var Default = Test
+
+// Test runs all tests with verbose output.
+func Test() error {
+	fmt.Println("Running tests...")
+	return sh.RunV("go", "test", "-v", "./...")
+}
+
+// Coverage runs tests with coverage reporting.
+func Coverage() error {
+	fmt.Println("Running tests with coverage...")
+	return sh.RunV("go", "test", "-coverprofile=coverage.out", "./...")
+}
+
+// Lint runs staticcheck for static analysis.
+func Lint() error {
+	fmt.Println("Running staticcheck...")
+	if _, err := exec.LookPath("staticcheck"); err != nil {
+		fmt.Println("staticcheck not found. Install with: go install honnef.co/go/tools/cmd/staticcheck@latest")
+		os.Exit(1)
+	}
+	return sh.RunV("staticcheck", "./...")
+}
+
+// Fmt formats all Go source files.
+func Fmt() error {
+	fmt.Println("Formatting Go files...")
+	return sh.RunV("gofmt", "-w", ".")
+}
+
+// Vet runs go vet for code analysis.
+func Vet() error {
+	fmt.Println("Running go vet...")
+	return sh.RunV("go", "vet", "./...")
+}
+
+// Check runs all quality checks: fmt, vet, staticcheck, and tests.
+func Check() {
+	mg.SerialDeps(Fmt, Vet, Lint, Test)
+}
+
+// Clean removes build artifacts.
+func Clean() error {
+	fmt.Println("Cleaning build artifacts...")
+	return sh.Rm("coverage.out")
+}