chore(hygiene): stop tracking env — the credentials were rotated by the client; the pattern was still teaching

Arkhos's audit (2026-09-02) closed the credential alarm: the client rotated every key, so the
blob in history is history, not exposure. What remained was the PATTERN — a file named `env`
under version control, in a repo other Origin sites read as a template.

Measured before changing anything:
- docker-compose.yml never references `env`; it substitutes ${VARS} from a git-ignored `.env`.
- The tracked `env` lacks DB_USER/DB_PASSWORD/DB_NAME that compose requires, so no deployment
  can be reading it. Untracking it deletes nothing a VPS `git pull` depends on.

Changes:
- `git rm --cached env` (history untouched; the dead values stay where they were).
- `.gitignore`: `env` (the existing `.env*` rule never matched a dotless name).
- `env.example`: every key compose substitutes, placeholder values only.
- README.md: the environment note Arkhos suggested, plus the deploy one-liner.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

env.example

@@ -0,0 +1,21 @@
+# Copy to `.env` (git-ignored). docker-compose.yml substitutes these ${VARS} — nothing reads a
+# file named `env`. Values are placeholders; never commit the real file.
+DB_USER=flux
+DB_PASSWORD=change-me
+DB_NAME=flux
+SESSION_SECRET=change-me-64-random-bytes
+OPENAI_API_KEY=sk-...
+NEXT_PUBLIC_APP_URL=https://rf-flux.com
+NEXT_PUBLIC_GA_ID=G-XXXXXXXXXX
+NEXT_PUBLIC_GSC_VERIFICATION=
+SMTP_HOST=smtp.example.com
+SMTP_PORT=587
+SMTP_USER=
+SMTP_PASS=
+SMTP_FROM=no-reply@rf-flux.com
+SMTP_SECURE=false
+# optional
+REDIS_URL=
+REDIS_TOKEN=
+BACKUP_RETENTION_DAYS=14
+BACKUP_INTERVAL_SECONDS=86400