feat: Fase 1-3 completas - precios proveedor, multi-sucursal, factura global

Fase 1: Lista de precios de proveedor
- Tabla supplier_catalog_prices en master DB
- Endpoints GET/POST/PUT/DELETE /supplier-catalog/prices
- Upload CSV/Excel de precios de proveedor
- Visualizacion de supplier_price en catalogo y POS

Fase 2: Multi-sucursal completo
- Migracion v4.0: inventory.branch_id=NULL, tabla inventory_stock
- Campos fiscales en branches (RFC, regimen, CP, serie CFDI, certificados)
- Trigger trg_update_inventory_stock para sincronizar stock por sucursal
- Backend config_bp.py con CRUD de sucursales fiscales
- Backend inventory_bp.py y pos_bp.py refactorizados para inventario compartido
- Backend invoicing_bp.py usa datos fiscales de la sucursal de la venta
- Frontend config.html/js con modal de sucursales expandido

Fase 3: Factura global mensual
- Migracion v4.1: tablas global_invoice_sales, sales.global_invoiced_at
- build_global_invoice_xml() con InformacionGlobal SAT-compliant
- Servicio global_invoice.py para agrupar ventas PUE <=000
- Endpoints POST/GET /global-invoice y /global-invoice/eligible-sales
- Frontend invoicing.html/js con boton y modal de factura global

pos/blueprints/pos_bp.py

@@ -15,6 +15,7 @@ from services.pos_engine import (
     process_sale, cancel_sale, calculate_totals,
     get_price_for_customer, get_margin_info
 )
+from services.inventory_engine import get_stock
 from services.audit import log_action
 from config import JWT_SECRET
 
@@ -34,7 +35,7 @@ def _enrich_items(cur, items, customer_id=None):
     # Batch fetch all inventory items in one query
     cur.execute("""
         SELECT id, part_number, name, cost, price_1, price_2, price_3,
-               tax_rate, branch_id
+               tax_rate
         FROM inventory WHERE id = ANY(%s) AND is_active = true
     """, (inv_ids,))
     inv_map = {r[0]: r for r in cur.fetchall()}
@@ -75,7 +76,6 @@ def _enrich_items(cur, items, customer_id=None):
             'unit_cost': float(inv[3]) if inv[3] else 0,
             'discount_pct': discount_pct,
             'tax_rate': tax_rate,
-            'branch_id': inv[8],
         })
     return enriched
 
@@ -103,6 +103,19 @@ def create_sale():
     data = request.get_json() or {}
     conn = get_tenant_conn(g.tenant_id)
 
+    # Verify stock availability per item for the active branch
+    branch_id = data.get('branch_id', g.branch_id)
+    for item in data.get('items', []):
+        inv_id = item.get('inventory_id')
+        qty = int(item.get('quantity', 1))
+        if inv_id:
+            available = get_stock(conn, inv_id, branch_id)
+            if available < qty:
+                conn.close()
+                return jsonify({
+                    'error': f'Insufficient stock for item {inv_id}. Available: {available}, requested: {qty}'
+                }), 400
+
     try:
         sale = process_sale(conn, data)
         conn.commit()