feat(pos): add gunicorn, marketplace B2B, and subscription billing (#7, #8, #12)

- Gunicorn production server with auto-scaled workers, run.sh, updated systemd service
- Marketplace B2B: cross-tenant inventory search, ordering, seller management with full UI
- Subscription billing: plan limits enforced on products/employees/branches, billing API + upgrade flow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/gunicorn.conf.py

@@ -0,0 +1,10 @@
+import multiprocessing
+
+bind = "0.0.0.0:5001"
+workers = multiprocessing.cpu_count() * 2 + 1
+worker_class = "sync"
+timeout = 120
+keepalive = 5
+accesslog = "/var/log/nexus-pos/access.log"
+errorlog = "/var/log/nexus-pos/error.log"
+loglevel = "info"