fix: validate required env vars at startup (#332)

[nomsoscript]

Closes #332

Changes

• Added a centralized required environment variable list in src/config.required.ts.
• Added startup validation that fails fast when any required env var is missing or empty.
• Kept the error descriptive so it names the missing variable(s).
• Updated config schema so empty PAYSTACK_PUBLIC_KEY is treated as unset, preserving its optional behavior.
• Added tests for required env validation and the optional Paystack key behavior.
• Aligned the startup optional-dependency warning tests with the actual optional config contract.

Testing

• corepack pnpm test -- src/config.required.test.ts src/config.test.ts src/utils/startup.utils.test.ts
• corepack pnpm build
• corepack pnpm lint
• prettier --check on the touched files

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