fix: pin meilisearch to v1.40.0

v1.42.1 (pulled by latest) is incompatible with existing data (db version
1.40.0). Pinning to v1.40.0 until a deliberate migration is planned.
Upgrade path: https://www.meilisearch.com/docs/learn/update_and_migration/updating

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -96,7 +96,7 @@ services:
 
   # ─── Meilisearch (full-text search) ──────────────────────────────────────────
   meilisearch:
-    image: getmeili/meilisearch:latest
+    image: getmeili/meilisearch:v1.40.0
     restart: unless-stopped
     environment:
       MEILI_MASTER_KEY: "${MEILI_MASTER_KEY}"