refactor(migration): delete pkg/migrator, move SQL to per-plugin embed

BREAKING: pkg/migrator/ deleted entirely. Migration SQL files are now
owned by each plugin in its own migrations/ directory.

Architecture:
- Delete pkg/migrator/ (26 global SQL files + ClickHouse migration)
- Move global SQL to per-plugin migrations/ with go:embed + Register()
- Rewrite cmd/app.go gooseEngine: uses Inject[DBService] for DB, iterates
  all plugin-registered MigrationEntry, runs goose.Up per entry
- core.MigrationEngine.Migrate signature changed: *Context instead of
  context.Context, so engine can resolve services via IoC

Per-plugin migration ownership:
  auth/             → w_access_tokens, w_auth_sources, w_external_accounts
  user/             → w_users (seed system user)
  admin/            → w_system_configs, w_templates (seeds)
  upload/           → w_uploads, w_upload_stats
  message_gateway/  → w_push_*, w_message_*
  risk_control/     → w_user_access_logs (PG + ClickHouse)
  driver_asynq_cron/  → w_schedules
  driver_asynq_worker/ → w_task_executions

Dependencies:
- cmd/banner.go: removed migration report display (migrations are automatic)
- cmd/reset_passwd.go: removed PreRun migrator.Migrate() call
- go.mod: clickhouse-go kept (used by plugins/infra/database/clickhouse.go)

plugins/domain/admin/plugin.go

@@ -6,6 +6,7 @@ package admin
 
 import (
 	"context"
+	"embed"
 
 	"github.com/Rain-kl/Wavelet/core"
 	"github.com/Rain-kl/Wavelet/core/contracts"
@@ -14,6 +15,9 @@ import (
 	"github.com/hibiken/asynq"
 )
 
+//go:embed migrations/*.sql
+var adminMigrations embed.FS
+
 // Option configures the admin plugin.
 type Option func(*Plugin)
 
@@ -56,6 +60,9 @@ func (p *Plugin) Apply(ctx *core.Context) error {
 	loginMW := authSvc.RequireAuthMiddleware().(gin.HandlerFunc)
 	adminMW := authSvc.RequireAdminMiddleware().(gin.HandlerFunc)
 
+	// 0a. Register migrations
+	ctx.Migrations().Register("admin", adminMigrations)
+
 	// 1. Register Admin HTTP Routes
 	adminRouter := ctx.Router().Group("/api/v1/admin", loginMW, adminMW)
 	{