feat: chunks recherche (snippets) + script migration

1. Recherche: fetchChunkSnippets() — après le classement RRF existant,
   récupère les passages précis qui matchent depuis NoteEmbeddingChunk.
   Pur affichage, AUCUN changement de classement.

2. Script migration: scripts/migrate-chunk-embeddings.ts
   Indexe toutes les notes existantes en fragments.
   Batch de 10, barre de progression.
   Usage: npx tsx scripts/migrate-chunk-embeddings.ts

3. Memory Echo chunk-level: à faire (US restante)

memento-note/scripts/migrate-chunk-embeddings.ts

@@ -0,0 +1,51 @@
+/**
+ * Script de migration : indexe toutes les notes existantes en fragments.
+ * Usage: npx tsx scripts/migrate-chunk-embeddings.ts
+ */
+import prisma from '@/lib/prisma'
+import { chunkIndexingService } from '@/lib/ai/services/chunk-indexing.service'
+
+async function main() {
+  console.log('=== Migration des chunks d\'embeddings ===\n')
+
+  const notes = await prisma.note.findMany({
+    where: {
+      trashedAt: null,
+      noteEmbedding: { isNot: null },
+    },
+    select: { id: true, title: true, content: true },
+    orderBy: { createdAt: 'asc' },
+  })
+
+  console.log(`${notes.length} notes à indexer\n`)
+
+  let done = 0
+  let errors = 0
+  const batchSize = 10
+
+  for (let i = 0; i < notes.length; i += batchSize) {
+    const batch = notes.slice(i, i + batchSize)
+
+    await Promise.all(batch.map(async (note) => {
+      try {
+        const result = await chunkIndexingService.indexNote(note.id, note.title, note.content)
+        if (result.newFragments > 0) {
+          done++
+        }
+      } catch (e: any) {
+        errors++
+        console.error(`  ✗ ${note.title?.slice(0, 40)}: ${e.message}`)
+      }
+    }))
+
+    process.stdout.write(`\r${Math.min(i + batchSize, notes.length)}/${notes.length} traitées`)
+  }
+
+  console.log(`\n\n=== Terminé ===`)
+  console.log(`Notes indexées: ${done}`)
+  console.log(`Erreurs: ${errors}`)
+
+  await prisma.$disconnect()
+}
+
+main().catch(console.error)