feat: validate required environment variables at startup (closes #7)

[TusharW4ni]

Problem (#7)

If a required env var is missing (e.g. a fresh clone with no .env), the app crashed with a confusing error deep inside the code — literally TypeError: Cannot read properties of undefined (reading 'replace') from server/utils/db.ts.

Fix

New server/utils/env.ts validates the required variables with Zod once, on import, and throws a clear message listing exactly what's missing:

Error: Invalid environment configuration:
  - DATABASE_URL: Invalid input: expected string, received undefined
  - BETTER_AUTH_SECRET: ...
  ...
Copy .env.example to .env and set the required values before starting the app.

db.ts and auth.ts consume the validated env, so validation runs before anything touches the database or auth.

Why a util and not a Nitro plugin

The issue suggested a server plugin, but db.ts reads DATABASE_URL at module load — earlier than any Nitro plugin runs — so a plugin would never get to report the problem (verified: the plugin approach still crashed with the old TypeError). Putting validation in a module that db.ts imports guarantees it runs first. Validation is skipped under Vitest (which runs without a real env).

Validated: DATABASE_URL, BETTER_AUTH_SECRET, BETTER_AUTH_URL, EMAIL_HOST, EMAIL_USER, EMAIL_PASS, EMAIL_FROM. UPLOAD_STORAGE_PATH is omitted (it has a safe code default).

Verification

• pnpm build succeeds; pnpm test passes (validation skipped under Vitest).
• Without env: built server exits fast (code 1) with the clear message above — before the old db crash.
• With env: built server starts, GET /api/health → 200.

Closes #7.

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