Research & choose blockchain framework

**Goal:** Pick the best tech stack for integrating blockchain into chamaa.api.

**Explanation:**
You must decide whether your backend will communicate with blockchain using:

- Ethers.js (Node-based) – great for TypeScript/Node APIs.

- Nethereum (.NET) – fits better if you’re using C#.

You also decide the blockchain network:

- Polygon (testnet: Mumbai) — ideal for low-cost testing.

- Ethereum Sepolia — for Ethereum-native testing.

**Deliverables:**

- Decision document (framework + reason).

- API keys for node provider (Alchemy, Infura, QuickNode).

- `.env` entries for `BLOCKCHAIN_RPC_URL`, `PRIVATE_KEY.`