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/package.json

@@ -14,7 +14,8 @@
     "db:migrate:deploy": "prisma migrate deploy",
     "db:push": "prisma db push",
     "db:studio": "prisma studio",
-    "db:reset": "prisma migrate reset",
+    "db:reset": "node -e \"console.error('BLOCKED: use prisma migrate deploy. See CLAUDE.md'); process.exit(1)\"",
+    "db:reset:unsafe": "prisma migrate reset",
     "db:switch": "node scripts/switch-db.js",
     "setup:env": "node scripts/setup-env.js",
     "test": "playwright test",