`.env.example` does not contain placeholder values in production templates

Values like STRIPE_SECRET_KEY=your_stripe_key_here or DATABASE_URL=TODO in .env.example waste every new contributor's onboarding time. They cannot tell which format the real value takes, whether it is a test key, or what character length to expect. The file exists specifically to bootstrap environments, so a placeholder that communicates nothing defeats its purpose and often leads developers to paste real credentials into the wrong slot.

Replace placeholder tokens with recognizable example values that preserve format and prefix conventions. Use public Stripe test-mode prefixes, a canonical Postgres URL shape, and documented character counts. Fix at .env.example:

STRIPE_SECRET_KEY=sk_test_xxxxxxxxxxxxxxxxxxxxxxxx
DATABASE_URL=postgresql://user:password@localhost:5432/dbname
NEXT_PUBLIC_SITE_URL=http://localhost:3000