Deployment guardrails and documentation alignment

The server now refuses to start when VAULT_ENV=production is set and VAULT_STORAGE_BACKEND is still local. Production deployments must use S3-compatible object storage. Local storage remains available for development without any change to that path.

Local compose verification was cleaned up. Development config checks can now run via make compose-check without requiring a real .env file on the local machine.

Documentation was aligned with current runtime behavior. Quotas and monthly limits are enforced by the API and reject requests when limits are exceeded — this was previously described incorrectly in several places. TLS wording was updated to reflect the actual deployment model: nginx terminates TLS, the FastAPI app binds to localhost only. Public pricing was also corrected to match the live checkout.