feat(backend): migrations Postgres testées (users, devices, resources, operations, ocr_jobs) + migrate au boot

- golang-migrate embarqué via embed (db/migrations/*.sql), MigrateDatabase(url) au boot
- ids TEXT 32-hex avec CHECK ~ '^[0-9a-f]{32}$' ; operations idempotentes UNIQUE(device_id, operation_id)
- harness db/migrations_test.go (up → assertions schéma → down, skip si PG indisponible)
- DB dev fraîche vaultdrop_dev (la DB vaultdrop héberge un prototype V2 abandonné)

docker-compose.yml

@@ -15,7 +15,7 @@ services:
     ports:
       - "5432:5432"
     environment:
-      POSTGRES_DB: vaultdrop
+      POSTGRES_DB: vaultdrop_dev
       POSTGRES_USER: vaultdrop
       POSTGRES_PASSWORD: vaultdrop
     volumes:
@@ -35,7 +35,7 @@ services:
   #     - "8080:8080"
   #   environment:
   #     - PORT=8080
-  #     - DATABASE_URL=postgres://vaultdrop:vaultdrop@postgres:5432/vaultdrop?sslmode=disable
+  #     - DATABASE_URL=postgres://vaultdrop:vaultdrop@postgres:5432/vaultdrop_dev?sslmode=disable
   #   volumes:
   #     - ./backend/uploads:/app/uploads
   #   depends_on: