Implement warehouse/lot pricing updates and configurator performance fixes

migrations/019_rename_stock_log_lot_to_partnumber.sql

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+ALTER TABLE stock_log
+    CHANGE COLUMN lot partnumber VARCHAR(255) NOT NULL;

migrations/020_add_only_in_stock_to_configurations.sql

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+-- Add only_in_stock toggle to configuration settings persisted in MariaDB.
+ALTER TABLE qt_configurations
+    ADD COLUMN IF NOT EXISTS only_in_stock BOOLEAN NOT NULL DEFAULT FALSE AFTER disable_price_refresh;

migrations/021_add_pricelist_items_pricelist_lot_index.sql

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+-- Ensure fast lookup for /api/quote/price-levels batched queries:
+-- SELECT ... FROM qt_pricelist_items WHERE pricelist_id = ? AND lot_name IN (...)
+SET @has_idx := (
+    SELECT COUNT(1)
+    FROM information_schema.statistics
+    WHERE table_schema = DATABASE()
+      AND table_name = 'qt_pricelist_items'
+      AND index_name IN ('idx_qt_pricelist_items_pricelist_lot', 'idx_pricelist_lot')
+);
+
+SET @ddl := IF(
+    @has_idx = 0,
+    'ALTER TABLE qt_pricelist_items ADD INDEX idx_qt_pricelist_items_pricelist_lot (pricelist_id, lot_name)',
+    'SELECT ''idx_qt_pricelist_items_pricelist_lot already exists, skip'''
+);
+
+PREPARE stmt FROM @ddl;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;