infra(gpu-box): commit GPU-Box compose to repo + Phase 2e docs

The GPU-Box stack has been carrying real production workload since
Phase 2c (monitoring) but only existed as a /srv/mana/docker-compose.gpu-box.yml
on the box itself. If the WSL filesystem dies, none of it is
reproducible. Bring the file into infrastructure/ as the source of
truth (live file on the box must be kept synchronous; manual rsync
for now since there's no CD into the GPU box).

Plus:
- infrastructure/.env.gpu-box.example as the secrets template
- infrastructure/README.md describing what runs there + how the
  Cloudflare-tunnel ingress is API-managed (not config.yml)
- .gitignore for the live infrastructure/.env.gpu-box copy
- MAC_MINI_SERVER.md status-page section now points at the GPU-Box
  setup instead of the long-stopped Mini container
- PLAN_OPTION_C.md: Phase 2e row + GPU-Box service tree update

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infrastructure/.env.gpu-box.example

@@ -0,0 +1,29 @@
+# GPU-Box stack secrets — copy to /srv/mana/.env on the Windows-WSL2 box,
+# never commit the filled version. Live-File ist gitignored.
+#
+# Verbraucher: docker-compose.gpu-box.yml in diesem Verzeichnis.
+
+# ─── Postgres-Credentials ────────────────────────────────────
+# Mini-Postgres-Passwort (gleiches wie .env.macmini POSTGRES_PASSWORD)
+# Wird von Forgejo + Umami genutzt, die ihren DB-Host auf 192.168.178.131:5432 zeigen.
+POSTGRES_PASSWORD=
+
+# ─── Grafana ─────────────────────────────────────────────────
+GF_ADMIN_PASSWORD=
+
+# ─── Umami ───────────────────────────────────────────────────
+# Identisch mit dem Wert auf dem Mini halten, sonst werden Sessions invalidiert.
+# Hexlich aus `openssl rand -base64 32`.
+UMAMI_APP_SECRET=
+
+# ─── Telegram-Notifier (alert-notifier) ──────────────────────
+TELEGRAM_BOT_TOKEN=
+TELEGRAM_CHAT_ID=
+NTFY_TOPIC=
+
+# ─── Glitchtip (eigene DB-Insel auf der GPU-Box) ─────────────
+# Frisch generiert bei Phase 2d, kein gemeinsamer Wert mit Mini.
+GLITCHTIP_DB_PASSWORD=
+GLITCHTIP_SECRET_KEY=
+GLITCHTIP_ADMIN_EMAIL=
+GLITCHTIP_ADMIN_PASSWORD=