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)

backend/config/config.go

@@ -33,7 +33,7 @@ func LoadApplicationConfig() (error, *ApplicationConfig) {
 
 	return nil, &ApplicationConfig{
 		Port:          port,
-		DatabaseURL:   get("DATABASE_URL", "postgres://vaultdrop:vaultdrop@localhost:5432/vaultdrop?sslmode=disable"),
+		DatabaseURL:   get("DATABASE_URL", "postgres://vaultdrop:vaultdrop@localhost:5432/vaultdrop_dev?sslmode=disable"),
 		UploadDir:     get("UPLOAD_DIR", "./uploads"),
 		MaxFileSizeMB: int64(maxSize),
 		OcrLang:       get("OCR_LANG", "fra+eng"),