fix: resolve unused variables in backend utils

Fixed @typescript-eslint/no-unused-vars errors in:
- starter.ts: removed unused error variables (2 fixes)
- auto-ssl-setup.ts: removed unused error variable (1 fix)
- ssh-key-utils.ts: removed unused error variables (3 fixes)
- user-crypto.ts: removed unused error variables (5 fixes)
- data-crypto.ts: removed unused plaintextFields and error variables (2 fixes)
- simple-db-ops.ts: removed unused parameters _userId and _tableName (2 fixes)

Total: 15 unused variable errors fixed

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

src/backend/utils/data-crypto.ts

@@ -97,7 +97,7 @@ class DataCrypto {
     let migratedFieldsCount = 0;
 
     try {
-      const { needsMigration, plaintextFields } =
+      const { needsMigration } =
         await LazyFieldEncryption.checkUserNeedsMigration(
           userId,
           userDataKey,
@@ -452,7 +452,7 @@ class DataCrypto {
       );
 
       return decrypted === testData;
-    } catch (error) {
+    } catch {
       return false;
     }
   }