Add CSV upload panel for meters and readings

- Add CSV upload service with upsert logic for meters
- Add CSV upload routes (POST /csv-upload/meters, POST /csv-upload/readings)
- Add template download endpoints for CSV format
- Create standalone upload-panel React app on port 5174
- Support concentrator_serial lookup for meter creation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

upload-panel/src/api/upload.ts

@@ -0,0 +1,65 @@
+const API_BASE_URL = import.meta.env.VITE_API_URL || 'http://localhost:3000/api';
+
+export interface UploadError {
+  row: number;
+  field?: string;
+  message: string;
+  data?: Record<string, unknown>;
+}
+
+export interface UploadResult {
+  total: number;
+  inserted: number;
+  updated: number;
+  errors: UploadError[];
+}
+
+export interface ApiResponse {
+  success: boolean;
+  message: string;
+  data?: UploadResult;
+}
+
+/**
+ * Upload meters CSV file
+ */
+export async function uploadMetersCSV(file: File): Promise<ApiResponse> {
+  const formData = new FormData();
+  formData.append('file', file);
+
+  const response = await fetch(`${API_BASE_URL}/csv-upload/meters`, {
+    method: 'POST',
+    body: formData,
+  });
+
+  return response.json();
+}
+
+/**
+ * Upload readings CSV file
+ */
+export async function uploadReadingsCSV(file: File): Promise<ApiResponse> {
+  const formData = new FormData();
+  formData.append('file', file);
+
+  const response = await fetch(`${API_BASE_URL}/csv-upload/readings`, {
+    method: 'POST',
+    body: formData,
+  });
+
+  return response.json();
+}
+
+/**
+ * Download meters CSV template
+ */
+export function downloadMetersTemplate(): void {
+  window.open(`${API_BASE_URL}/csv-upload/meters/template`, '_blank');
+}
+
+/**
+ * Download readings CSV template
+ */
+export function downloadReadingsTemplate(): void {
+  window.open(`${API_BASE_URL}/csv-upload/readings/template`, '_blank');
+}