feat: add environment configuration and validation with Zod (#2)

[temisan0x]

Closes #2

What Changed

• Added src/config/env.ts with a Zod schema that validates environment variables at startup and exits with clear field-level errors when validation fails.
• Updated src/config/index.ts to import validated values from env.ts and export a typed config object.
• Updated src/config/health.ts to use validated env values instead of raw process.env.
• Updated src/index.ts to use config for PORT, UPSTREAM_URL, and PROXY_TIMEOUT_MS.
• Added .env.example with all supported environment variables and placeholder values.
• Updated README.md with a full environment variable table (required/optional, defaults, and descriptions).
• Updated .gitignore to keep .env ignored while allowing .env.example to be committed (!.env.example).

Validation Behavior

• Required vars fail fast on boot:
  • JWT_SECRET
  • ADMIN_API_KEY
  • METRICS_API_KEY
• Conditional requirements fail fast on boot:
  • SOROBAN_RPC_ENABLED=true requires SOROBAN_RPC_URL
  • HORIZON_ENABLED=true requires HORIZON_URL

Testing

• Missing required vars -> app exits immediately with clear validation errors listing each missing field.
• Enabled optional integrations without required URLs -> app exits immediately with clear conditional validation errors.
• Valid env (from .env / DOTENV_CONFIG_PATH) -> app starts cleanly on the configured port.

Scope Note

• Existing process.env usage outside the env-config scope of this issue was intentionally left unchanged.

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