feat(05-03): add user-plugin organisations table (structure only)

- create_organisations folds PHP v3.2.0; no users-table ALTER this phase
- Organisation model is an FK target; app.key placeholder in config

models/organisation.go

@@ -0,0 +1,20 @@
+package models
+
+import "time"
+
+// Organisation is the structure-only FK target for Fonoteka org credentials.
+// Roles and membership behavior belong to Phase 7 AUTH-02.
+type Organisation struct {
+	ID          uint      `gorm:"column:id;primaryKey"`
+	Slug        string    `gorm:"column:slug"`
+	Name        string    `gorm:"column:name"`
+	Description *string   `gorm:"column:description"`
+	CreatedAt   time.Time `gorm:"column:created_at"`
+	UpdatedAt   time.Time `gorm:"column:updated_at"`
+}
+
+func (Organisation) TableName() string { return "golem15_user_organisations" }
+
+func init() {
+	Register(Organisation{})
+}