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)

core/app.go

@@ -212,7 +212,7 @@ func (a *App) ApplyPlugins() error {
 }
 
 // RunMigrations dispatches migration execution across all registered plugin migration entries.
-func (a *App) RunMigrations(ctx context.Context) error {
+func (a *App) RunMigrations() error {
 	entries := a.ctx.Migrations().Entries()
 	if len(entries) == 0 {
 		return nil
@@ -233,7 +233,7 @@ func (a *App) RunMigrations(ctx context.Context) error {
 		return nil
 	}
 
-	if err := engine.Migrate(ctx, entries); err != nil {
+	if err := engine.Migrate(a.ctx, entries); err != nil {
 		return fmt.Errorf("core: migration failed: %w", err)
 	}
 
@@ -275,7 +275,7 @@ func (a *App) Start(ctx ...context.Context) error {
 	}
 
 	// 2. Run migrations
-	if err := a.RunMigrations(baseCtx); err != nil {
+	if err := a.RunMigrations(); err != nil {
 		a.mu.Lock()
 		a.running = false
 		a.mu.Unlock()