FASE 7b: DB Performance — Pooling, Stock Summary, N+1 fix

Cambios implementados:

1. Connection pooling (tenant_db.py):
   - psycopg2.pool.ThreadedConnectionPool para master y tenants
   - Wrapper _PooledConnection que devuelve al pool en .close()
   - Cero cambios en blueprints (backward compatible)

2. Tabla inventory_stock_summary + triggers (v3.2):
   - O(1) stock lookup en vez de SUM() sobre historial completo
   - Trigger AFTER INSERT en inventory_operations recalcula stock
   - Poblada inicialmente en ambos tenants
   - Refactor en 6 archivos de servicios para usar la nueva tabla

3. Fix N+1 en process_sale (pos_engine.py):
   - Precarga retail_price en bulk query FOR UPDATE
   - Elimina SELECT individual por item en loop

4. Índices críticos:
   - idx_parts_name_part + pattern_ops (master)
   - idx_inv_ops_inventory_branch_created (tenants)
   - idx_wi_part_stock_positive (master, ya existía desde Fase 1)

Tests: 73/73 pasando (compat + fase3 + fase5 + fase6)
Migración: v3.2_db_performance.sql

pos/services/ai_chat.py

@@ -140,7 +140,7 @@ def get_inventory_context(tenant_conn, branch_id=None):
         cur.execute(f"""
             SELECT COUNT(*) FROM inventory i
             WHERE {where}
-              AND COALESCE((SELECT SUM(quantity) FROM inventory_operations WHERE inventory_id = i.id), 0) <= 3
+              AND COALESCE((SELECT stock FROM inventory_stock_summary WHERE inventory_id = i.id), 0) <= 3
         """, params)
         low_stock = cur.fetchone()[0] or 0