fix: clean up orphan labels when syncing note labels

After syncing a note's labels, detect and delete labels in the same
notebook that are no longer referenced by any note (neither via JSON
nor labelRelations). Prevents phantom labels from accumulating.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/app/actions/notes.ts

@@ -134,7 +134,8 @@ async function syncLabels(userId: string, noteLabels: string[] = [], notebookId?
   }
 }
 
-/** Sync both Note.labels (JSON) AND labelRelations for a single note. */
+/** Sync both Note.labels (JSON) AND labelRelations for a single note.
+ *  Also cleans up orphan labels in the same notebook scope. */
 async function syncNoteLabels(noteId: string, labelNames: string[], notebookId: string | null, userId: string) {
   const uniqueNames = [...new Set(labelNames.map(n => n.trim()).filter(Boolean))]
   const labelRows = await syncLabels(userId, uniqueNames, notebookId)
@@ -146,6 +147,46 @@ async function syncNoteLabels(noteId: string, labelNames: string[], notebookId:
       labelRelations: { set: labelIds.map(id => ({ id })) },
     },
   })
+
+  // Clean up orphan labels: labels in this notebook scope that are no longer
+  // referenced by any note (neither via JSON nor via labelRelations)
+  if (notebookId !== null) {
+    const allLabels = await prisma.label.findMany({
+      where: { notebookId, userId },
+      select: { id: true, name: true },
+    })
+    if (allLabels.length > 0) {
+      const labelIdsSet = new Set(allLabels.map(l => l.id))
+      // Find notes in this notebook that have any label relation
+      const notesWithLabels = await prisma.note.findMany({
+        where: {
+          notebookId,
+          userId,
+          labelRelations: { some: { id: { in: Array.from(labelIdsSet) } } },
+        },
+        select: { labels: true },
+      })
+      // Collect all label names still in use via JSON
+      const namesInUse = new Set<string>()
+      for (const n of notesWithLabels) {
+        if (n.labels) {
+          try {
+            const parsed = JSON.parse(n.labels as string)
+            if (Array.isArray(parsed)) {
+              parsed.filter((x: any) => typeof x === 'string').forEach((x: string) => namesInUse.add(x.toLowerCase()))
+            }
+          } catch {}
+        }
+      }
+      // Delete labels not in use
+      const orphans = allLabels.filter(l => !namesInUse.has(l.name.toLowerCase()))
+      if (orphans.length > 0) {
+        await prisma.label.deleteMany({
+          where: { id: { in: orphans.map(l => l.id) } },
+        })
+      }
+    }
+  }
 }
 
 /** Après déplacement via API : rattacher les étiquettes de la note au bon carnet */