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

pkg/util/gin_context.go

@@ -0,0 +1,22 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import "github.com/gin-gonic/gin"
+
+// GetFromContext retrieves a typed value from Gin context.
+func GetFromContext[T any](c *gin.Context, key string) (T, bool) {
+	value, exists := c.Get(key)
+	if !exists {
+		var zero T
+		return zero, false
+	}
+	typed, ok := value.(T)
+	return typed, ok
+}
+
+// SetToContext sets a typed value into Gin context.
+func SetToContext[T any](c *gin.Context, key string, value T) {
+	c.Set(key, value)
+}