feat(pos): add barcode generator with PostgreSQL sequence

pos/migrations/v1.0_initial.sql

@@ -351,3 +351,6 @@ CREATE INDEX idx_audit_log_employee ON audit_log(employee_id);
 CREATE INDEX idx_audit_log_created ON audit_log(created_at);
 CREATE UNIQUE INDEX idx_inventory_branch_part ON inventory(branch_id, part_number);
 CREATE INDEX idx_employee_sessions_token ON employee_sessions(token);
+
+-- Barcode sequence
+CREATE SEQUENCE IF NOT EXISTS barcode_seq START 1;

pos/services/barcode_generator.py

@@ -0,0 +1,61 @@
+# /home/Autopartes/pos/services/barcode_generator.py
+"""Generate internal barcodes for parts that don't have manufacturer barcodes.
+Format: NX-{tenant_short}-{sequential_number}
+
+Uses a PostgreSQL sequence (barcode_seq) per tenant schema to guarantee uniqueness
+under concurrent requests. No MAX+1 race conditions.
+"""
+
+
+def _ensure_sequence(conn):
+    """Create the barcode sequence if it doesn't exist yet."""
+    cur = conn.cursor()
+    cur.execute("""
+        DO $$
+        BEGIN
+            IF NOT EXISTS (
+                SELECT 1 FROM pg_class WHERE relkind = 'S' AND relname = 'barcode_seq'
+            ) THEN
+                CREATE SEQUENCE barcode_seq START WITH 1 INCREMENT BY 1 NO MAXVALUE;
+            END IF;
+        END $$;
+    """)
+    cur.close()
+
+
+def generate_barcode(conn, tenant_db_name):
+    """Generate the next barcode for this tenant.
+
+    Format: NX-{tenant_short}-{NNNNNNN}
+    Example: NX-REFLOPEZ-0000001
+
+    Uses a PostgreSQL sequence to guarantee unique sequential numbers even
+    under concurrent requests (no race conditions).
+    """
+    short = tenant_db_name.replace('tenant_', '').replace('_', '').upper()[:10]
+    prefix = f"NX-{short}-"
+
+    _ensure_sequence(conn)
+
+    cur = conn.cursor()
+    cur.execute("SELECT nextval('barcode_seq')")
+    next_num = cur.fetchone()[0]
+    cur.close()
+
+    return f"{prefix}{next_num:07d}"
+
+
+def generate_barcodes_batch(conn, tenant_db_name, count):
+    """Generate multiple sequential barcodes atomically."""
+    short = tenant_db_name.replace('tenant_', '').replace('_', '').upper()[:10]
+    prefix = f"NX-{short}-"
+
+    _ensure_sequence(conn)
+
+    cur = conn.cursor()
+    # setval + generate range atomically via a single call
+    cur.execute("SELECT nextval('barcode_seq') FROM generate_series(1, %s)", (count,))
+    nums = [r[0] for r in cur.fetchall()]
+    cur.close()
+
+    return [f"{prefix}{n:07d}" for n in nums]