[P3] [docs] .env.example does not document the state of SERVER_PRIVATE_KEY

[AndreaDiazCorreia]

Labels: docs, priority:low

Severity: LOW
Area: config

Files:

• .env.example

Problem:
The env var appears in the example with no note that it is inert until Phase 4.

Proposal:

• Add an inline comment explaining that it is inert in Phase 3 and how to generate it.

Acceptance criteria:

• .env.example has the comment.