feat: add env.mjs for centralized env validation (t3-env)

Validates all required env vars at runtime with zod schemas.
Skips validation during build via SKIP_ENV_VALIDATION flag.
Replaces scattered process.env access with typed env imports.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

turbo.json

@@ -7,6 +7,7 @@
       "inputs": ["$TURBO_DEFAULT$", ".env*"],
       "outputs": [".next/**", "!.next/cache/**", "dist/**"],
       "env": [
+        "SKIP_ENV_VALIDATION",
         "BETTER_AUTH_SECRET",
         "BETTER_AUTH_URL",
         "GOOGLE_CLIENT_ID",