docs(config): document and validate environment

[therealbibson]

Closes #248

• Document every runtime environment variable in .env.example without real credentials.
• Validate required database URLs and conditional mainnet RPC configuration before loading application modules; report all missing values together.
• Add tests for env documentation coverage and startup validation behavior.

Validation: targeted env tests and TypeScript build pass. The full npm test run encounters the pre-existing OpenAPI snapshot mismatch noted in #248 and did not complete before hanging.

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