feat(07-01): add QuickAddItem migration, model, and query functions

- Create 003_quick_add_library.sql with quick_add_items table and user index
- Add QuickAddItem struct to models.go following Category pattern
- Add ListQuickAddItems, CreateQuickAddItem, UpdateQuickAddItem, DeleteQuickAddItem to queries.go

backend/migrations/003_quick_add_library.sql

@@ -0,0 +1,11 @@
+CREATE TABLE quick_add_items (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    name VARCHAR(100) NOT NULL,
+    icon VARCHAR(50) NOT NULL DEFAULT '',
+    sort_order INT NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_quick_add_items_user ON quick_add_items(user_id);