Validate required environment variables at startup and document per-network config

[priscaenoch]

Problem

.env.example defines large per-network variable groups (TESTNET_*, MAINNET_*, DEVNET_* for database, read replica, RPC, RPC WS, Horizon, passphrase, API subdomain) plus indexer tuning vars. There is no fail-fast validation, so a missing or misnamed variable surfaces as a deep runtime error on the first request that needs it rather than at boot. zod is already a dependency and is well suited to this.

What needs to be done

• Add a startup config validator (using zod) that checks all required variables for the selected network (STELLAR_NETWORK/NODE_ENV) and exits with a clear message listing missing/invalid vars.
• Apply it to both the API service (src/index.ts) and the indexer entrypoint.
• Document the required variables per network in the README, and confirm the TESTNET_* set matches the live testnet deployment.

Files

• new src/config/validateEnv.ts (and indexer equivalent)
• src/index.ts, indexer/src/index.js/config.js
• .env.example, README.md

Acceptance deliverables

• Starting either service with a missing required variable fails immediately with a clear, actionable message.
• README documents required variables per network.

Tests to pass

• Unit test: validator passes with a complete env and throws listing the missing keys with an incomplete env.