Improve .env.example: mark required vs optional, tidy comments

[nbkdoesntknowcoding]

.env.example is the first thing a self-hoster copies — make it easier to fill in correctly.

Task: without changing any variable names or default values, improve the annotations: clearly mark which vars are required vs optional, group related vars under short section headers, and fix stale/confusing comments. Keep it consistent with the README quickstart.

Files: .env.example

Acceptance criteria

• Every var is labeled required or optional.
• No variable names/defaults changed (behavior identical).
• Reads cleanly top-to-bottom for a first-time self-hoster.

Size: S

[nbkdoesntknowcoding]

.env.example now marks required vs optional and was tidied in #65 (payment placeholders removed). Closing.