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).

static/js/app-detail.js

@@ -73,11 +73,30 @@ document.addEventListener("alpine:init", () => {
                 body: JSON.stringify(
                     this.vars.map((e) => ({ key: e.key, value: e.value })),
                 ),
-            }).then((res) => {
-                if (res.ok) {
-                    window.location.reload();
-                }
-            });
+            })
+                .then((res) => {
+                    if (res.ok) {
+                        window.location.reload();
+                        return;
+                    }
+                    res.json()
+                        .then((data) => {
+                            window.alert(
+                                data.error ||
+                                    "Failed to save environment variables.",
+                            );
+                        })
+                        .catch(() => {
+                            window.alert(
+                                "Failed to save environment variables.",
+                            );
+                        });
+                })
+                .catch(() => {
+                    window.alert(
+                        "Network error: could not save environment variables.",
+                    );
+                });
         },
     }));