fix: transactional env var save, empty key validation, frontend error handling

- Wrap DELETE + INSERTs in a database transaction via new
  ReplaceEnvVarsByAppID() to prevent silent data loss on partial
  insert failure. Rollback on any error; return 500 instead of 200.
- Add server-side validation rejecting entries with empty keys
  (returns 400 with error message).
- Add frontend error handling for non-2xx responses with user-visible
  alert messages.
- Remove stale //nolint:dupl directives (files no longer duplicate).

internal/models/env_var.go

@@ -1,4 +1,3 @@
-//nolint:dupl // Active Record pattern - similar structure to label.go is intentional
 package models
 
 import (
@@ -139,3 +138,49 @@ func DeleteEnvVarsByAppID(
 
 	return err
 }
+
+// EnvVarPair is a key-value pair for bulk env var operations.
+type EnvVarPair struct {
+	Key   string
+	Value string
+}
+
+// ReplaceEnvVarsByAppID atomically replaces all env vars for an app
+// within a single database transaction. It deletes all existing env
+// vars and inserts the provided pairs. If any operation fails, the
+// entire transaction is rolled back.
+func ReplaceEnvVarsByAppID(
+	ctx context.Context,
+	db *database.Database,
+	appID string,
+	pairs []EnvVarPair,
+) error {
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("beginning transaction: %w", err)
+	}
+
+	defer func() { _ = tx.Rollback() }()
+
+	_, err = tx.ExecContext(ctx, "DELETE FROM app_env_vars WHERE app_id = ?", appID)
+	if err != nil {
+		return fmt.Errorf("deleting env vars: %w", err)
+	}
+
+	for _, p := range pairs {
+		_, err = tx.ExecContext(ctx,
+			"INSERT INTO app_env_vars (app_id, key, value) VALUES (?, ?, ?)",
+			appID, p.Key, p.Value,
+		)
+		if err != nil {
+			return fmt.Errorf("inserting env var %q: %w", p.Key, err)
+		}
+	}
+
+	err = tx.Commit()
+	if err != nil {
+		return fmt.Errorf("committing transaction: %w", err)
+	}
+
+	return nil
+}

internal/models/label.go

@@ -1,4 +1,3 @@
-//nolint:dupl // Active Record pattern - similar structure to env_var.go is intentional
 package models
 
 import (