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)

cmd/banner.go

@@ -14,14 +14,11 @@ import (
 
 	"github.com/Rain-kl/Wavelet/pkg/buildinfo"
 	"github.com/Rain-kl/Wavelet/pkg/config"
-	"github.com/Rain-kl/Wavelet/pkg/migrator"
 )
 
 //nolint:unused // startup banner formatting utilities
 type startupState struct {
 	mode           string
-	relationalDB   migrator.Report
-	clickHouseDB   migrator.Report
 	listensForHTTP bool
 }
 
@@ -42,8 +39,6 @@ func formatStartupBanner(state startupState) string {
 		fmt.Sprintf(" Environment: %s", config.Config.App.Env),
 		fmt.Sprintf(" Runtime:     %s/%s (%s)", runtime.GOOS, runtime.GOARCH, runtime.Version()),
 		fmt.Sprintf(" Build time:  %s", buildTime()),
-		fmt.Sprintf(" Database:    %s", formatMigration(state.relationalDB)),
-		fmt.Sprintf(" Analytics:   %s", formatMigration(state.clickHouseDB)),
 	}
 	if state.listensForHTTP {
 		lines = append(lines, fmt.Sprintf(" Listening:   http://%s", config.Config.App.Addr))
@@ -57,15 +52,4 @@ func buildTime() string {
 		return "development build"
 	}
 	return buildinfo.BuildTime
-}
-
-func formatMigration(report migrator.Report) string {
-	if !report.Enabled {
-		return "disabled"
-	}
-	state := "up to date"
-	if report.Applied {
-		state = "upgraded"
-	}
-	return fmt.Sprintf("%s (version %d, %s)", report.Backend, report.Version, state)
-}
+}