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/peer_service.py

@@ -93,11 +93,7 @@ def get_local_inventory(tenant_conn, query: str = None, limit: int = 50) -> list
                COALESCE(s.stock, 0) AS stock,
                i.unit, i.catalog_part_id
         FROM inventory i
-        LEFT JOIN (
-            SELECT inventory_id, SUM(quantity) AS stock
-            FROM inventory_operations
-            GROUP BY inventory_id
-        ) s ON s.inventory_id = i.id
+        LEFT JOIN inventory_stock_summary s ON s.inventory_id = i.id
         WHERE {where}
         ORDER BY i.name
         LIMIT %s