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/core/config/feature-tiers.ts

@@ -36,6 +36,11 @@ export const FEATURE_TIERS: Record<string, FeatureConfig> = {
     name: 'Receipt Scan',
     upgradePrompt: 'Upgrade to Pro to scan fuel receipts and auto-fill your fuel log entries.',
   },
+  'maintenance.receiptScan': {
+    minTier: 'pro',
+    name: 'Maintenance Receipt Scan',
+    upgradePrompt: 'Upgrade to Pro to scan maintenance receipts and extract service details automatically.',
+  },
 } as const;
 
 /**