feat: add expiry date and low-stock threshold columns (#63 #67)

supabase/migrations/006_add_expiry_lowstock.sql

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+-- Migration: Add expiry date and low-stock threshold tracking
+-- Issues: #63 (expiry tracking), #67 (low-stock threshold)
+-- Created: 2026-02-25
+
+-- Add expires_at column for expiry date tracking
+ALTER TABLE pantry_items
+ADD COLUMN expires_at TIMESTAMP WITH TIME ZONE DEFAULT NULL;
+
+-- Add low_stock_threshold column for low-stock alerts
+ALTER TABLE pantry_items
+ADD COLUMN low_stock_threshold NUMERIC(10,2) DEFAULT NULL;
+
+-- Add comments for documentation
+COMMENT ON COLUMN pantry_items.expires_at IS 'Optional expiration date for the item. Null means no expiry tracking.';
+COMMENT ON COLUMN pantry_items.low_stock_threshold IS 'Minimum quantity threshold. Item is considered low-stock when quantity <= threshold. Null means no threshold set.';
+
+-- Create index for efficient expiry queries (finding items expiring soon)
+CREATE INDEX idx_pantry_items_expires_at ON pantry_items(expires_at) WHERE expires_at IS NOT NULL;
+
+-- Create index for efficient low-stock queries
+CREATE INDEX idx_pantry_items_low_stock ON pantry_items(quantity, low_stock_threshold) 
+WHERE low_stock_threshold IS NOT NULL;