Integrate Ethereum (Polygon testnet)

### Objective
Integrate the Polygon (Ethereum Layer 2) blockchain network to enable on-chain transactions for ChamaaChain’s savings and governance features.

### Why Polygon?
Polygon offers fast, low-cost, and eco-friendly transactions—perfect for micro-savings and DAO governance in emerging markets.

### Key Tasks
- Configure Polygon testnet (Mumbai) connection using **Alchemy** or **Infura**.
- Deploy a simple **smart contract** for on-chain savings records.
- Implement a service (via **Ethers.js** or **Nethereum**) to interact with the contract.
- Store transaction hashes in the backend DB (PostgreSQL).

### Tech Stack
- **Smart Contracts**: Solidity
- **Integration**: Ethers.js / Nethereum
- **Blockchain Network**: Polygon (Mumbai Testnet)
- **Backend**: NestJS API

### Acceptance Criteria
- [ ] Successful deployment of the smart contract on Polygon testnet.
- [ ] Backend can read/write on-chain transactions.
- [ ] Each user’s transaction hash is stored and retrievable from the database.
- [ ] Tested end-to-end with mock savings and governance data.

### References
- [Polygon Docs](https://docs.polygon.technology/)
- [Ethers.js Guide](https://docs.ethers.io/v5/)
- [Alchemy Quickstart](https://docs.alchemy.com/)

### Future Extension
After testnet validation, migrate to **Polygon mainnet** and expand smart contracts to include **loan approval and voting modules**.