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/services/billing.py

@@ -0,0 +1,74 @@
+"""Subscription billing for Nexus POS SaaS."""
+
+from tenant_db import get_master_conn
+
+PLANS = {
+    'free':       {'name': 'Gratis',     'price_mxn': 0,    'max_products': 100,   'max_employees': 2,    'max_branches': 1},
+    'basic':      {'name': 'Basico',     'price_mxn': 499,  'max_products': 5000,  'max_employees': 5,    'max_branches': 2},
+    'pro':        {'name': 'Pro',        'price_mxn': 1499, 'max_products': 50000, 'max_employees': 15,   'max_branches': 5},
+    'enterprise': {'name': 'Enterprise', 'price_mxn': 3999, 'max_products': None,  'max_employees': None, 'max_branches': None},
+}
+
+PLAN_ORDER = ['free', 'basic', 'pro', 'enterprise']
+
+
+def get_plan(tenant_id):
+    """Get current plan for a tenant."""
+    conn = get_master_conn()
+    cur = conn.cursor()
+    cur.execute("SELECT plan FROM tenants WHERE id = %s", (tenant_id,))
+    row = cur.fetchone()
+    cur.close()
+    conn.close()
+    if not row:
+        return 'free'
+    plan_key = row[0] or 'free'
+    if plan_key not in PLANS:
+        plan_key = 'free'
+    return plan_key
+
+
+def get_plan_details(tenant_id):
+    """Get plan key + full details for a tenant."""
+    plan_key = get_plan(tenant_id)
+    return {**PLANS[plan_key], 'key': plan_key}
+
+
+def next_plan(current_plan):
+    """Return the next upgrade plan key, or None if already enterprise."""
+    idx = PLAN_ORDER.index(current_plan) if current_plan in PLAN_ORDER else 0
+    if idx < len(PLAN_ORDER) - 1:
+        return PLAN_ORDER[idx + 1]
+    return None
+
+
+def check_limit(tenant_id, resource, current_count):
+    """Check if tenant is within plan limits.
+
+    Args:
+        tenant_id: Tenant ID
+        resource: One of 'max_products', 'max_employees', 'max_branches'
+        current_count: Current count of that resource
+
+    Returns:
+        (allowed: bool, limit: int|None, current: int)
+    """
+    plan_key = get_plan(tenant_id)
+    plan = PLANS[plan_key]
+    limit = plan.get(resource)
+    if limit is None:
+        return (True, None, current_count)
+    return (current_count < limit, limit, current_count)
+
+
+def upgrade_plan(tenant_id, new_plan):
+    """Change tenant's plan."""
+    if new_plan not in PLANS:
+        return {'error': f'Invalid plan: {new_plan}'}
+    conn = get_master_conn()
+    cur = conn.cursor()
+    cur.execute("UPDATE tenants SET plan = %s WHERE id = %s", (new_plan, tenant_id))
+    conn.commit()
+    cur.close()
+    conn.close()
+    return {'success': True, 'plan': new_plan, 'details': PLANS[new_plan]}