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/tenant_db.py

@@ -1,20 +1,85 @@
 # /home/Autopartes/pos/tenant_db.py
-"""Tenant DB connection manager. Gets a psycopg2 connection for a specific tenant."""
+"""Tenant DB connection manager with pooling.
+
+Uses psycopg2.pool.ThreadedConnectionPool for both master and tenant DBs.
+Connections are returned to the pool on .close() via a thin wrapper —
+this keeps the rest of the codebase unchanged.
+"""
 
 import psycopg2
+from psycopg2 import pool
 from config import MASTER_DB_URL, TENANT_DB_URL_TEMPLATE
 
 
+# ─── Pools ─────────────────────────────────────
+_master_pool = None
+_tenant_pools = {}
+
+
+def _get_master_pool():
+    """Lazy-initialize master DB connection pool."""
+    global _master_pool
+    if _master_pool is None:
+        _master_pool = pool.ThreadedConnectionPool(
+            minconn=2, maxconn=10, dsn=MASTER_DB_URL
+        )
+    return _master_pool
+
+
+def _get_tenant_pool(db_name):
+    """Lazy-initialize tenant DB connection pool by db_name."""
+    global _tenant_pools
+    if db_name not in _tenant_pools:
+        dsn = TENANT_DB_URL_TEMPLATE.format(db_name=db_name)
+        _tenant_pools[db_name] = pool.ThreadedConnectionPool(
+            minconn=2, maxconn=10, dsn=dsn
+        )
+    return _tenant_pools[db_name]
+
+
+class _PooledConnection:
+    """Thin wrapper that delegates all attribute access to the real
+    psycopg2 connection, but returns it to the pool on .close().
+    """
+    __slots__ = ('_conn', '_pool')
+
+    def __init__(self, conn, db_pool):
+        self._conn = conn
+        self._pool = db_pool
+
+    def __getattr__(self, name):
+        return getattr(self._conn, name)
+
+    def close(self):
+        try:
+            self._pool.putconn(self._conn)
+        except Exception:
+            # If pool is already closed, fall back to real close
+            self._conn.close()
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.close()
+
+
+# ─── Public API ────────────────────────────────
+
 def get_master_conn():
-    """Get connection to nexus_master DB."""
-    return psycopg2.connect(MASTER_DB_URL)
+    """Get a pooled connection to the master DB."""
+    p = _get_master_pool()
+    return _PooledConnection(p.getconn(), p)
 
 
 def get_tenant_conn(tenant_id):
-    """Get connection to a tenant's DB by looking up db_name in nexus_master."""
+    """Get a pooled connection to a tenant's DB."""
     master = get_master_conn()
     cur = master.cursor()
-    cur.execute("SELECT db_name FROM tenants WHERE id = %s AND is_active = true", (tenant_id,))
+    cur.execute(
+        "SELECT db_name FROM tenants WHERE id = %s AND is_active = true",
+        (tenant_id,)
+    )
     row = cur.fetchone()
     cur.close()
     master.close()
@@ -23,9 +88,11 @@ def get_tenant_conn(tenant_id):
         raise ValueError(f"Tenant {tenant_id} not found or inactive")
 
     db_name = row[0]
-    return psycopg2.connect(TENANT_DB_URL_TEMPLATE.format(db_name=db_name))
+    p = _get_tenant_pool(db_name)
+    return _PooledConnection(p.getconn(), p)
 
 
 def get_tenant_conn_by_dbname(db_name):
-    """Get connection to a tenant DB directly by name."""
-    return psycopg2.connect(TENANT_DB_URL_TEMPLATE.format(db_name=db_name))
+    """Get a pooled connection to a tenant DB directly by name."""
+    p = _get_tenant_pool(db_name)
+    return _PooledConnection(p.getconn(), p)