feat: add backend migration and API for maintenance receipt linking (refs #151)

Add receipt_document_id FK on maintenance_records, update types/repo/service
to support receipt linking on create and return document metadata on GET.
Add OCR proxy endpoint POST /api/ocr/extract/maintenance-receipt with
tier gating (maintenance.receiptScan) through full chain: routes -> controller
-> service -> client.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/ocr/external/ocr-client.ts

@@ -177,6 +177,57 @@ export class OcrClient {
     return result;
   }
 
+  /**
+   * Extract data from a maintenance receipt image using maintenance-specific OCR.
+   *
+   * @param fileBuffer - Image file buffer
+   * @param contentType - MIME type of the file
+   * @returns Receipt extraction result (receiptType: "maintenance")
+   */
+  async extractMaintenanceReceipt(
+    fileBuffer: Buffer,
+    contentType: string
+  ): Promise<ReceiptExtractionResponse> {
+    const formData = this.buildFormData(fileBuffer, contentType);
+    const url = `${this.baseUrl}/extract/maintenance-receipt`;
+
+    logger.info('OCR maintenance receipt extract request', {
+      operation: 'ocr.client.extractMaintenanceReceipt',
+      url,
+      contentType,
+      fileSize: fileBuffer.length,
+    });
+
+    const response = await this.fetchWithTimeout(url, {
+      method: 'POST',
+      body: formData,
+    });
+
+    if (!response.ok) {
+      const errorText = await response.text();
+      logger.error('OCR maintenance receipt extract failed', {
+        operation: 'ocr.client.extractMaintenanceReceipt.error',
+        status: response.status,
+        error: errorText,
+      });
+      const err: any = new Error(`OCR service error: ${response.status} - ${errorText}`);
+      err.statusCode = response.status;
+      throw err;
+    }
+
+    const result = (await response.json()) as ReceiptExtractionResponse;
+
+    logger.info('OCR maintenance receipt extract completed', {
+      operation: 'ocr.client.extractMaintenanceReceipt.success',
+      success: result.success,
+      receiptType: result.receiptType,
+      fieldCount: Object.keys(result.extractedFields).length,
+      processingTimeMs: result.processingTimeMs,
+    });
+
+    return result;
+  }
+
   /**
    * Submit an async OCR job for large files.
    *