security: reject weak/default PASTE_AUTH_SECRET in production

[ArianAr]

Problem

Compose defaults PASTE_AUTH_SECRET to the public placeholder change-me-in-production-use-a-long-random-string. Production starts because any non-empty secret is accepted. Attackers who know the default can forge pc_auth_<id> HMAC unlock cookies and read password-protected pastes without the password.

Acceptance

• Production refuses empty and known placeholder secrets
• Compose does not ship a working default secret (require explicit env)
• Docs/README still show how to set a strong secret
• Unit tests for getPasteAuthSecret placeholder rejection

Test plan

• pnpm test / lint / typecheck / build