better handle importing of existing local repository config + documentation

README.md

@@ -151,7 +151,21 @@ Secrets/credentials in the config file can reference environment variables using
     "compressionMode": "auto"
   }
   ```
-  > **Note for importing existing local repositories:** If you're importing an existing repository (e.g., from a backup or migration), include the `name` field in `config` with the original subfolder name. The actual restic repo is stored at `{path}/{name}`. You can find this value in an exported config under `repositories[].config.name`.
+  > **Note for importing existing local repositories:** If you're migrating an existing repository (e.g., from a backup or another Zerobyte instance), include the `name` field in `config` with the original subfolder name, and set `isExistingRepository: true`. The actual restic repo is stored at `{path}/{name}`.
+  >
+  > **Example (migration):**
+  > ```json
+  > {
+  >   "name": "my-local-repo",
+  >   "config": {
+  >     "backend": "local",
+  >     "path": "/var/lib/zerobyte/repositories",
+  >     "name": "abc123",
+  >     "isExistingRepository": true
+  >   }
+  > }
+  > ```
+  > You can find the `config.name` value in an exported config under `repositories[].config.name`. This value must be unique across all repositories.
 
 - **S3-Compatible**
   ```json

app/server/modules/repositories/repositories.service.ts

@@ -73,7 +73,20 @@ const createRepository = async (name: string, config: RepositoryConfig, compress
 
 	const id = crypto.randomUUID();
 
-	const shortId = (config.backend === "local" && config.name) ? config.name : generateShortId();
+	// Determine shortId: use provided config.name for local repo migrations, otherwise generate
+	let shortId: string;
+	if (config.backend === "local" && config.name?.length) {
+		// User provided a name (migration scenario) - check for conflicts
+		shortId = config.name;
+		const existingByShortId = await db.query.repositoriesTable.findFirst({
+			where: eq(repositoriesTable.shortId, shortId),
+		});
+		if (existingByShortId) {
+			throw new ConflictError(`A repository with shortId '${shortId}' already exists. The shortId is used as the subdirectory name for local repositories.`);
+		}
+	} else {
+		shortId = generateShortId();
+	}
 
 	let processedConfig = config;
 	if (config.backend === "local") {