Startup validation for all required environment variables

[phertyameen]

Location

src/config/

Problem

The README lists required env vars for the local sandbox test explicitly, implying no confirmed centralized startup validation exists ensuring the app fails fast with a clear error rather than failing confusingly deep in a request handler when a var is missing or malformed.

Acceptance Criteria

• App refuses to boot with a clear, specific error listing every missing/invalid required env var at once
• Covers database, Stellar/Soroban, and security-related (JWT_SECRET etc.) variables at minimum
• Validation logic itself is unit tested, not just assumed correct