fix(12.2): close code-review blockers on uploads, JSON caps, and pivot fill

Keep form save behind in-flight uploads, make retries idempotent via X-Upload-Id, cap remaining JSON bodies, and surface pending pivot type errors instead of zeroing them.

Co-authored-by: Cursor <cursoragent@cursor.com>

modules/lagoon/deferred.go

@@ -69,8 +69,9 @@ func (k DeferredKey) validate() error {
 // holds the pivot values of a deferred belongsToMany link. A binding without
 // the envelope (nil pivot_data, or JSON without these keys) is a plain link.
 type DeferredEnvelope struct {
-	Created bool           `json:"created,omitempty"`
-	Pivot   map[string]any `json:"pivot,omitempty"`
+	Created  bool           `json:"created,omitempty"`
+	Pivot    map[string]any `json:"pivot,omitempty"`
+	UploadID string         `json:"upload_id,omitempty"`
 }
 
 // Envelope decodes the binding's pivot_data. Nil or empty pivot_data is the
@@ -167,7 +168,7 @@ func insertBinding(ctx context.Context, tx *gorm.DB, key DeferredKey, field, sla
 		IsBind:        bind,
 		BackendUserID: key.AdminID,
 	}
-	if env != nil && (env.Created || len(env.Pivot) > 0) {
+	if env != nil && (env.Created || len(env.Pivot) > 0 || env.UploadID != "") {
 		raw, err := json.Marshal(env)
 		if err != nil {
 			return fmt.Errorf("lagoon: deferred binding envelope: %w", err)