Add sub-categories to affirmation categories

Add self-referential parent/child relation on AffirmationCategory for
   one-level hierarchy. Update AffirmationForm with two-level category and
   sub-category dropdowns, server actions with find-or-create hierarchy
   logic, AffirmationList with Parent

prisma/migrations/20260223163626_add_category_parent/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "AffirmationCategory" ADD COLUMN     "parentId" INTEGER;
+
+-- AddForeignKey
+ALTER TABLE "AffirmationCategory" ADD CONSTRAINT "AffirmationCategory_parentId_fkey" FOREIGN KEY ("parentId") REFERENCES "AffirmationCategory"("id") ON DELETE SET NULL ON UPDATE CASCADE;

prisma/schema.prisma

@@ -128,8 +128,11 @@ model SolfeggioFrequency {
 }
 
 model AffirmationCategory {
-  id           Int           @id @default(autoincrement())
-  name         String        @unique
+  id           Int                   @id @default(autoincrement())
+  name         String                @unique
+  parentId     Int?
+  parent       AffirmationCategory?  @relation("CategoryHierarchy", fields: [parentId], references: [id])
+  children     AffirmationCategory[] @relation("CategoryHierarchy")
   affirmations Affirmation[]
 }