security: .env.example contains real RSA private key and worker secret as example values

Description

The .env.example file contains what appears to be a real RSA private key
and worker secret used as example values rather than placeholder strings.

Specifically:

• Line with WORKER_RUNS_PRIVATE_KEY contains a base64-encoded value that
  decodes to a valid -----BEGIN RSA PRIVATE KEY----- block
• Line with WORKER_SECRET contains a real 256-bit secret value

Risk

Any operator who copies these values directly into their production .env
without generating new keys will be running with publicly known credentials.
An attacker with access to this repo can use these keys to authenticate as
a worker to any Lightning instance using the example values.

Suggested Fix

Replace the real key material with generation instructions:

# WORKER_RUNS_PRIVATE_KEY="generate with: mix lightning.gen_worker_keys"
# WORKER_SECRET="generate with: mix lightning.gen_worker_keys"

The DEPLOYMENT.md already documents mix lightning.gen_worker_keys —
pointing to it from the .env.example is safer than providing real keys.

Steps to Verify

echo "LS0tLS1CRUdJTiBSU0EgUFJJVkFURSBLRVktLS0tLQ..." | base64 -d
# Returns a valid RSA private key

Inspect the credential entries in .env.example, then read DEPLOYMENT.md for the documented mix lightning.gen_worker_keys command. Replace the exposed example values with generation guidance that points to that command, and verify that the example contains no usable key or secret.