feat(transactions): implement sync, categorization, and history in TransactionsService

[coderDom-x]

## Summary Implements the four core `TransactionsService` methods per #108: `syncTransactions`, `getHistory`, `categorize`, and `getSpendingByCategory`. Horizon integration uses `@stellar/stellar-sdk`'s `Horizon.Server`, matching the pattern already established in `BlockchainService`.

Changes

• syncTransactions(userId, publicKey): fetches the last 200 Horizon operations, filters to payment-type ops, normalizes into TransactionEntity rows, inserts via ON CONFLICT (hash) DO NOTHING, returns the count of rows actually inserted (via RETURNING, so duplicates aren't counted)
• getHistory(userId, options): paginated query with optional category/asset filters, ordered newest-first
• categorize(userId, transactionId, category): updates category on a single row; throws NotFoundException if missing, ForbiddenException if it belongs to another user
• getSpendingByCategory(userId, asset, since): aggregated totals per category
• Unit tests in transactions.service.spec.ts covering sync deduplication (including repeat-call idempotency), pagination, category ownership checks, and aggregation

Not included in this PR

• TransactionsController wiring with JwtAuthGuard
• transactions.module.ts TypeORM wiring
• New index migration (add-transactions-indexes.ts)

These remain open scope items from #108 — this PR covers the service layer only. Will follow up separately / update if handling the rest in this same branch.

Testing

• npm run build — clean, no errors
• npm test -- transactions —
  closes Implement transaction sync, categorization, and history in TransactionsService #108

[Uyoxy]

Nice work — this fully closes out #108. Went through each acceptance
criterion:

• ✅ syncTransactions: ON CONFLICT dedup verified, including the
  "run twice, no duplicate rows" test case
• ✅ getHistory: pagination + category/asset filters
• ✅ categorize: correct 403/404 ownership handling
• ✅ getSpendingByCategory: aggregation query
• ✅ Controller wired with JwtAuthGuard on all four routes
• ✅ Module updated for TypeORM
• ✅ Index migration present for (user_id, created_at DESC) and
  (user_id, category)
• ✅ Unit tests cover sync dedup, pagination, ownership, aggregation
  please fix ci issues so i can merge

[Uyoxy]

great