Add brand history tracking for new/dropped brands detection

- Add first_seen_at/last_seen_at columns to brands table
- Create brand_history table for event tracking (added, dropped, returned)
- Add dispensary_brand_stats view for dashboard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/migrations/027_brand_history_tracking.sql

@@ -0,0 +1,57 @@
+-- Migration 027: Brand History Tracking
+-- Add first_seen_at and last_seen_at to brands table for history tracking
+-- Also add dispensary_id since brands is currently store_id linked
+
+-- Add timestamp columns to brands
+ALTER TABLE brands ADD COLUMN IF NOT EXISTS first_seen_at TIMESTAMPTZ DEFAULT NOW();
+ALTER TABLE brands ADD COLUMN IF NOT EXISTS last_seen_at TIMESTAMPTZ DEFAULT NOW();
+ALTER TABLE brands ADD COLUMN IF NOT EXISTS dispensary_id INTEGER REFERENCES dispensaries(id);
+
+-- Add index for tracking dropped brands
+CREATE INDEX IF NOT EXISTS idx_brands_last_seen ON brands(last_seen_at DESC);
+CREATE INDEX IF NOT EXISTS idx_brands_dispensary ON brands(dispensary_id);
+
+-- Create brand_history table for detailed tracking
+CREATE TABLE IF NOT EXISTS brand_history (
+  id SERIAL PRIMARY KEY,
+  dispensary_id INTEGER NOT NULL REFERENCES dispensaries(id) ON DELETE CASCADE,
+  brand_name VARCHAR(255) NOT NULL,
+  event_type VARCHAR(20) NOT NULL, -- 'added', 'dropped', 'returned'
+  event_at TIMESTAMPTZ DEFAULT NOW(),
+  product_count INTEGER, -- number of products from this brand at event time
+  metadata JSONB
+);
+
+CREATE INDEX IF NOT EXISTS idx_brand_history_dispensary ON brand_history(dispensary_id, event_at DESC);
+CREATE INDEX IF NOT EXISTS idx_brand_history_brand ON brand_history(brand_name, event_at DESC);
+CREATE INDEX IF NOT EXISTS idx_brand_history_event ON brand_history(event_type, event_at DESC);
+
+-- Create view for brand stats per dispensary
+CREATE OR REPLACE VIEW dispensary_brand_stats AS
+SELECT
+  d.id AS dispensary_id,
+  COALESCE(d.dba_name, d.name) AS dispensary_name,
+  COUNT(DISTINCT p.brand) FILTER (WHERE p.last_seen_at >= NOW() - INTERVAL '7 days') AS current_brands,
+  COUNT(DISTINCT p.brand) AS total_brands_ever,
+  (
+    SELECT COUNT(DISTINCT brand_name)
+    FROM brand_history bh
+    WHERE bh.dispensary_id = d.id
+    AND bh.event_type = 'added'
+    AND bh.event_at >= NOW() - INTERVAL '7 days'
+  ) AS new_brands_7d,
+  (
+    SELECT COUNT(DISTINCT brand_name)
+    FROM brand_history bh
+    WHERE bh.dispensary_id = d.id
+    AND bh.event_type = 'dropped'
+    AND bh.event_at >= NOW() - INTERVAL '7 days'
+  ) AS dropped_brands_7d
+FROM dispensaries d
+LEFT JOIN products p ON p.dispensary_id = d.id
+GROUP BY d.id, d.dba_name, d.name;
+
+-- Grant permissions
+GRANT SELECT ON brand_history TO scraper;
+GRANT INSERT, UPDATE ON brand_history TO scraper;
+GRANT SELECT ON dispensary_brand_stats TO scraper;