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/app.py

@@ -51,6 +51,9 @@ def create_app():
     from blueprints.whatsapp_bp import whatsapp_bp
     app.register_blueprint(whatsapp_bp)
 
+    from blueprints.marketplace_bp import marketplace_bp
+    app.register_blueprint(marketplace_bp)
+
     # Health check
     @app.route('/pos/health')
     def health():
@@ -113,6 +116,10 @@ def create_app():
     def pos_whatsapp():
         return render_template('whatsapp.html')
 
+    @app.route('/pos/marketplace')
+    def pos_marketplace():
+        return render_template('marketplace.html')
+
     @app.route('/pos/static/<path:filename>')
     def pos_static(filename):
         return send_from_directory('static', filename)