[BACKEND] Add a `GET /wallet` summary endpoint

[Trovic1]

Summary

Return balance plus aggregate totals (sent, received, transaction count) in one call so the dashboard can avoid multiple round-trips.

Scope

• backend only

Likely files

• src/routes/WalletRoutes.js
• src/models/Transaction.js, src/models/Wallet.js

Acceptance criteria

• New authenticated route returns balance + totals
• Totals computed correctly in both storage modes
• Existing /wallet/balance remains for compatibility

Verification

• Compare endpoint output to the existing per-call values

Labels: backend, enhancement, help wanted

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