feat(transactions): implement sync, history, categorize, spending aggregation

- syncTransactions: fetch last 200 Horizon ops, normalize payments,
  insert with ON CONFLICT (hash) DO NOTHING, return new-row count
- getHistory: paginated query with optional category/asset filters
- categorize: updates category, enforces userId ownership (403/404)
- getSpendingByCategory: aggregated totals grouped by category
- Fix Horizon SDK import to use @stellar/stellar-sdk Horizon.Server,
  matching BlockchainService
- Add unit tests covering dedup, pagination, ownership checks, aggregation

Refs #108

Author: coderDom-x

package-lock.json

@@ -46,7 +46,7 @@
     "joi": "17.13.5",
     "jsonwebtoken": "9.0.3",
     "nest-winston": "1.10.2",
-    "nodemailer": "7.0.13",
+    "nodemailer": "^9.0.5",
     "passport": "0.7.0",
     "passport-jwt": "4.0.1",
     "passport-local": "1.0.0",