handle merging tag categories

src/app/api/tags/categories/[id]/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server'
-import { updateCategory, deleteCategory, deleteCategoryForce, getTags } from '@/lib/tags'
+import { updateCategory, deleteCategory, deleteCategoryForce, getTags, getCategories, mergeCategories } from '@/lib/tags'
 import { requireAdmin } from '@/lib/auth'
 
 export async function PATCH(
@@ -11,9 +11,30 @@ export async function PATCH(
 
   try {
     const { id } = await params
-    const { name } = await request.json()
-    const category = updateCategory(id, name)
-    return NextResponse.json(category)
+    const { name, merge } = await request.json()
+
+    try {
+      const category = updateCategory(id, name)
+      return NextResponse.json(category)
+    } catch (err) {
+      const msg = (err as Error).message
+      if (!msg.includes('already exists')) throw err
+
+      // A category with this name already exists — find it
+      const trimmed = (name as string).trim()
+      const target = getCategories().find((c) => c.name.toLowerCase() === trimmed.toLowerCase())
+      if (!target) throw err
+
+      if (merge) {
+        mergeCategories(id, target.id)
+        return NextResponse.json(target)
+      }
+
+      return NextResponse.json(
+        { error: msg, conflict: true, targetCategoryId: target.id },
+        { status: 409 }
+      )
+    }
   } catch (err) {
     return NextResponse.json({ error: (err as Error).message }, { status: 400 })
   }