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/migrations/v3.9_supplier_catalog_prices.sql

@@ -0,0 +1,40 @@
+-- v3.9_supplier_catalog_prices.sql
+-- Per-tenant supplier pricing for items in the master supplier_catalog.
+-- This table lives in the master DB and is joined by tenant_id.
+
+CREATE TABLE IF NOT EXISTS supplier_catalog_prices (
+    id              SERIAL PRIMARY KEY,
+    tenant_id       INTEGER NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+    catalog_id      INTEGER NOT NULL REFERENCES supplier_catalog(id) ON DELETE CASCADE,
+    price           NUMERIC(12,2) NOT NULL,
+    currency        VARCHAR(3) DEFAULT 'MXN',
+    effective_from  DATE DEFAULT CURRENT_DATE,
+    effective_to    DATE,
+    is_active       BOOLEAN DEFAULT TRUE,
+    created_at      TIMESTAMPTZ DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ DEFAULT NOW(),
+    UNIQUE(tenant_id, catalog_id, effective_from)
+);
+
+CREATE INDEX IF NOT EXISTS idx_supplier_catalog_prices_tenant_catalog
+    ON supplier_catalog_prices(tenant_id, catalog_id, effective_from DESC)
+    WHERE is_active = true;
+
+-- Index for quick "latest active price" lookups per tenant+item.
+CREATE INDEX IF NOT EXISTS idx_supplier_catalog_prices_lookup
+    ON supplier_catalog_prices(tenant_id, catalog_id, effective_from DESC, is_active);
+
+-- Trigger to keep updated_at current on row changes.
+CREATE OR REPLACE FUNCTION update_supplier_catalog_prices_updated_at()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = NOW();
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_supplier_catalog_prices_updated_at ON supplier_catalog_prices;
+CREATE TRIGGER trg_supplier_catalog_prices_updated_at
+    BEFORE UPDATE ON supplier_catalog_prices
+    FOR EACH ROW
+    EXECUTE FUNCTION update_supplier_catalog_prices_updated_at();