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/migrations/v1.6_marketplace.sql

@@ -0,0 +1,32 @@
+-- Marketplace orders (cross-tenant)
+-- These tables go in nexus_autoparts (master DB), not tenant DBs
+
+CREATE TABLE IF NOT EXISTS marketplace_orders (
+    id SERIAL PRIMARY KEY,
+    buyer_tenant_id INTEGER NOT NULL,
+    seller_tenant_id INTEGER NOT NULL,
+    buyer_name VARCHAR(200),
+    seller_name VARCHAR(200),
+    total NUMERIC(12,2),
+    status VARCHAR(20) DEFAULT 'pending', -- pending, confirmed, shipped, delivered, cancelled
+    notes TEXT,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS marketplace_order_items (
+    id SERIAL PRIMARY KEY,
+    order_id INTEGER REFERENCES marketplace_orders(id),
+    part_number VARCHAR(100),
+    part_name VARCHAR(300),
+    quantity INTEGER,
+    unit_price NUMERIC(12,2),
+    subtotal NUMERIC(12,2)
+);
+
+CREATE INDEX IF NOT EXISTS idx_mp_orders_buyer ON marketplace_orders(buyer_tenant_id);
+CREATE INDEX IF NOT EXISTS idx_mp_orders_seller ON marketplace_orders(seller_tenant_id);
+CREATE INDEX IF NOT EXISTS idx_mp_orders_status ON marketplace_orders(status);
+
+-- Add is_seller flag to tenants table
+ALTER TABLE tenants ADD COLUMN IF NOT EXISTS is_seller BOOLEAN DEFAULT FALSE;