fix(database): add Scan method to insert query interfaces

* Implement Scan method for BunInsertQuery, GormInsertQuery, and PgSQLInsertQuery
* Update mock implementations to support Scan method
* Introduce GetForeignKeyColumn utility for foreign key resolution
* Add tests for GetForeignKeyColumn functionality

pkg/reflection/model_utils_foreign_key_test.go

@@ -0,0 +1,123 @@
+package reflection
+
+import (
+	"reflect"
+	"testing"
+)
+
+// --- local test models ---
+
+type fkDept struct{}
+
+// bunEmployee uses bun join: tag to declare the FK column explicitly.
+type bunEmployee struct {
+	DeptID     string   `bun:"dept_id"                          json:"dept_id"`
+	Department *fkDept  `bun:"rel:belongs-to,join:dept_id=id"   json:"department"`
+}
+
+// gormEmployee uses gorm foreignKey: tag (mirrors testmodels.Employee).
+type gormEmployee struct {
+	DepartmentID string  `json:"department_id"`
+	ManagerID    string  `json:"manager_id"`
+	Department   *fkDept `gorm:"foreignKey:DepartmentID;references:ID" json:"department"`
+	Manager      *fkDept `gorm:"foreignKey:ManagerID;references:ID"    json:"manager"`
+}
+
+// conventionEmployee has no explicit FK tag — relies on naming convention.
+type conventionEmployee struct {
+	DepartmentID string  `json:"department_id"`
+	Department   *fkDept `json:"department"`
+}
+
+// noTagEmployee has a relation field with no FK tag and no convention match.
+type noTagEmployee struct {
+	Unrelated *fkDept `json:"unrelated"`
+}
+
+func TestGetForeignKeyColumn(t *testing.T) {
+	tests := []struct {
+		name      string
+		modelType reflect.Type
+		parentKey string
+		want      string
+	}{
+		// Bun join: tag
+		{
+			name:      "bun join tag returns local column",
+			modelType: reflect.TypeOf(bunEmployee{}),
+			parentKey: "department",
+			want:      "dept_id",
+		},
+		{
+			name:      "bun join tag matched via json tag (case-insensitive)",
+			modelType: reflect.TypeOf(bunEmployee{}),
+			parentKey: "Department",
+			want:      "dept_id",
+		},
+
+		// GORM foreignKey: tag
+		{
+			name:      "gorm foreignKey resolves to column name",
+			modelType: reflect.TypeOf(gormEmployee{}),
+			parentKey: "department",
+			want:      "department_id",
+		},
+		{
+			name:      "gorm foreignKey resolves second relation",
+			modelType: reflect.TypeOf(gormEmployee{}),
+			parentKey: "manager",
+			want:      "manager_id",
+		},
+		{
+			name:      "gorm foreignKey matched case-insensitively",
+			modelType: reflect.TypeOf(gormEmployee{}),
+			parentKey: "Department",
+			want:      "department_id",
+		},
+
+		// Pointer and slice unwrapping
+		{
+			name:      "pointer to struct is unwrapped",
+			modelType: reflect.TypeOf(&gormEmployee{}),
+			parentKey: "department",
+			want:      "department_id",
+		},
+		{
+			name:      "slice of struct is unwrapped",
+			modelType: reflect.TypeOf([]gormEmployee{}),
+			parentKey: "department",
+			want:      "department_id",
+		},
+
+		// No tag — returns "" so caller can fall back to convention
+		{
+			name:      "relation with no FK tag returns empty string",
+			modelType: reflect.TypeOf(conventionEmployee{}),
+			parentKey: "department",
+			want:      "",
+		},
+
+		// Unknown parent key
+		{
+			name:      "unknown parent key returns empty string",
+			modelType: reflect.TypeOf(gormEmployee{}),
+			parentKey: "nonexistent",
+			want:      "",
+		},
+		{
+			name:      "non-struct type returns empty string",
+			modelType: reflect.TypeOf(""),
+			parentKey: "department",
+			want:      "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := GetForeignKeyColumn(tt.modelType, tt.parentKey)
+			if got != tt.want {
+				t.Errorf("GetForeignKeyColumn(%v, %q) = %q, want %q", tt.modelType, tt.parentKey, got, tt.want)
+			}
+		})
+	}
+}