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/inventory_engine.py

@@ -25,15 +25,33 @@ def _safe_g(attr, default=None):
 def get_stock(conn, inventory_id, branch_id=None):
     """Get current stock for an inventory item. Optionally filter by branch.
 
-    Uses Redis cache first, falls back to PostgreSQL SUM query.
+    Uses Redis cache first, then inventory_stock_summary, falls back to
+    PostgreSQL SUM query.
     """
     # Try Redis first
     cached = get_cached_stock(inventory_id, branch_id)
     if cached is not None:
         return cached
 
-    # Fallback to PostgreSQL
+    # Use inventory_stock_summary (O(1) lookup)
     cur = conn.cursor()
+    if branch_id:
+        cur.execute(
+            "SELECT stock FROM inventory_stock_summary WHERE inventory_id = %s AND branch_id = %s",
+            (inventory_id, branch_id)
+        )
+    else:
+        cur.execute(
+            "SELECT stock FROM inventory_stock_summary WHERE inventory_id = %s",
+            (inventory_id,)
+        )
+    row = cur.fetchone()
+    if row is not None:
+        cur.close()
+        set_cached_stock(inventory_id, row[0], branch_id)
+        return row[0]
+
+    # Fallback to PostgreSQL SUM (legacy, should not reach here if trigger works)
     if branch_id:
         cur.execute(
             "SELECT COALESCE(SUM(quantity), 0) FROM inventory_operations WHERE inventory_id = %s AND branch_id = %s",
@@ -55,21 +73,17 @@ def get_stock(conn, inventory_id, branch_id=None):
 def get_stock_bulk(conn, branch_id=None):
     """Get stock for all items. Returns dict {inventory_id: stock_quantity}.
 
-    Uses PostgreSQL directly (bulk operation, Redis wouldn't help much here
-    unless we pre-populated all keys).
+    Uses inventory_stock_summary for O(1) bulk lookup.
     """
     cur = conn.cursor()
     if branch_id:
         cur.execute("""
-            SELECT inventory_id, COALESCE(SUM(quantity), 0)
-            FROM inventory_operations WHERE branch_id = %s
-            GROUP BY inventory_id
+            SELECT inventory_id, stock
+            FROM inventory_stock_summary WHERE branch_id = %s
         """, (branch_id,))
     else:
         cur.execute("""
-            SELECT inventory_id, COALESCE(SUM(quantity), 0)
-            FROM inventory_operations
-            GROUP BY inventory_id
+            SELECT inventory_id, stock FROM inventory_stock_summary
         """)
     stock_map = {r[0]: r[1] for r in cur.fetchall()}
     cur.close()