refactor(core): fix cross-domain auth imports, unify migration, add downstream scaffold

Architecture:
- Move GetFromContext/SetToContext from plugins/domain/auth to pkg/util
- Move auth context key constants to core/contracts (AuthUserObjKey, AuthTokenAuthKey, etc.)
- Add AuthUserIDKey, AuthUserNameKey, GetCurrentUserID, RevokeToken to contracts.AuthService
- All 4 domain plugin Apply() methods now resolve AuthService via core.Using IoC
- Plugin route middleware uses authSvc.RequireAuthMiddleware() cast to gin.HandlerFunc
- DisallowTokenAuth added to AuthService contract

Migration:
- Replace cmd/app.go SetMigrationRunner bridge with gooseEngine implementing core.MigrationEngine
- Remove cmd/root.go PreRun migration hooks and runMigrations() function
- Migrations now run via core.App.Start() → RunMigrations()

Events:
- Add complete domain event topic catalog and payload DTOs to core/contracts/events.go
- 15 event topics across auth, user, admin, upload, message_gateway, risk_control

Downstream:
- Create downstream/ directory with README and custom_example plugin scaffold

CI:
- Update Makefile code-check architecture guards for Cordis layering
- Enforce: core no gin/gorm/asynq, contracts no plugins/, pkg no plugins/, domain no cross-domain

cmd/app.go

@@ -8,8 +8,8 @@ import (
 	"time"
 
 	"github.com/Rain-kl/Wavelet/core"
-	"github.com/Rain-kl/Wavelet/core/extpoints"
 	"github.com/Rain-kl/Wavelet/pkg/config"
+	"github.com/Rain-kl/Wavelet/pkg/migrator"
 	"github.com/Rain-kl/Wavelet/plugins/domain/admin"
 	"github.com/Rain-kl/Wavelet/plugins/domain/auth"
 	"github.com/Rain-kl/Wavelet/plugins/domain/cap"
@@ -56,11 +56,8 @@ func newWaveletApp(profile core.Profile) *core.App {
 		system.New(),
 	)
 
-	// 3. Bind Goose migration runner
-	app.SetMigrationRunner(func(_ context.Context, _ []extpoints.MigrationEntry) error {
-		runMigrations()
-		return nil
-	})
+	// 3. Bind Goose migration engine (wraps pkg/migrator.Migrate)
+	app.SetMigrationEngine(&gooseEngine{})
 
 	// 4. Mount runtime drivers for each aspect
 	app.Use(
@@ -71,3 +68,12 @@ func newWaveletApp(profile core.Profile) *core.App {
 
 	return app
 }
+
+// gooseEngine implements core.MigrationEngine by wrapping pkg/migrator.Migrate.
+// It runs all centrally managed Goose SQL migrations during app startup.
+type gooseEngine struct{}
+
+func (e *gooseEngine) Migrate(_ context.Context, _ []core.MigrationEntry) error {
+	_ = migrator.Migrate()
+	return nil
+}