Add sales tracking system with customers, analytics, and inventory management

- Add customers table (021) and sales/sale_items tables (022) migrations
- Add customer CRUD, sale CRUD (transactional with auto inventory decrement/restore),
  and analytics API endpoints (overview, top sellers, revenue chart, inventory alerts)
- Add sales page with NewSaleModal (customer autocomplete, multi-item form,
  color-based pricing, stock validation) and SaleDetailModal
- Add customers page with search, inline editing, and purchase history
- Add analytics dashboard with recharts (revenue line chart, top sellers bar,
  refill vs spulna pie chart, inventory alerts table with stockout estimates)
- Add customerService, saleService, analyticsService to frontend API layer
- Update sidebar navigation on all admin pages

database/migrations/022_add_sales_tables.sql

@@ -0,0 +1,24 @@
+-- Add sales and sale_items tables for tracking transactions
+CREATE TABLE sales (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    customer_id UUID REFERENCES customers(id) ON DELETE SET NULL,
+    total_amount INTEGER NOT NULL DEFAULT 0,
+    notes TEXT,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE sale_items (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    sale_id UUID NOT NULL REFERENCES sales(id) ON DELETE CASCADE,
+    filament_id UUID NOT NULL REFERENCES filaments(id) ON DELETE RESTRICT,
+    item_type VARCHAR(10) NOT NULL CHECK (item_type IN ('refill', 'spulna')),
+    quantity INTEGER NOT NULL DEFAULT 1 CHECK (quantity > 0),
+    unit_price INTEGER NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_sales_customer_id ON sales (customer_id);
+CREATE INDEX idx_sales_created_at ON sales (created_at);
+CREATE INDEX idx_sale_items_sale_id ON sale_items (sale_id);
+CREATE INDEX idx_sale_items_filament_id ON sale_items (filament_id);