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_test.go

@@ -9,7 +9,6 @@ import (
 
 	"github.com/Rain-kl/Wavelet/pkg/buildinfo"
 	"github.com/Rain-kl/Wavelet/pkg/config"
-	"github.com/Rain-kl/Wavelet/pkg/migrator"
 )
 
 func TestFormatStartupBanner(t *testing.T) {
@@ -30,14 +29,7 @@ func TestFormatStartupBanner(t *testing.T) {
 	config.Config.App.Addr = ":3000"
 
 	banner := formatStartupBanner(startupState{
-		mode: "API",
-		relationalDB: migrator.Report{
-			Backend: "PostgreSQL",
-			Enabled: true,
-			Version: 202607150003,
-			Applied: true,
-		},
-		clickHouseDB:   migrator.Report{Backend: "ClickHouse"},
+		mode:           "API",
 		listensForHTTP: true,
 	})
 
@@ -45,8 +37,6 @@ func TestFormatStartupBanner(t *testing.T) {
 		"Wavelet v3.2.1",
 		"Environment: production",
 		"Build time:  2026-07-13T08:00:00Z",
-		"Database:    PostgreSQL (version 202607150003, upgraded)",
-		"Analytics:   disabled",
 		"Listening:   http://:3000",
 		"Mode:        API",
 	} {
@@ -54,4 +44,4 @@ func TestFormatStartupBanner(t *testing.T) {
 			t.Errorf("banner missing %q:\n%s", want, banner)
 		}
 	}
-}
+}