Add store selection to API permissions

- Add store_id and store_name columns to wp_dutchie_api_permissions
- Backend: Add /stores endpoint, require store_id when creating permissions
- Frontend: Add store selector dropdown to API Permissions form
- WordPress plugin v1.3.0: Remove store_id from shortcodes (store is tied to token)

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

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

backend/migrations/029_add_store_id_to_wp_api_permissions.sql

@@ -0,0 +1,13 @@
+-- Migration: Add store_id to wp_dutchie_api_permissions
+-- This allows API tokens to be associated with a specific store
+
+-- Add store_id column to wp_dutchie_api_permissions
+ALTER TABLE wp_dutchie_api_permissions
+ADD COLUMN IF NOT EXISTS store_id INTEGER REFERENCES stores(id);
+
+-- Add index for faster lookups
+CREATE INDEX IF NOT EXISTS idx_wp_api_permissions_store_id ON wp_dutchie_api_permissions(store_id);
+
+-- Add store_name column to return store info without join
+ALTER TABLE wp_dutchie_api_permissions
+ADD COLUMN IF NOT EXISTS store_name VARCHAR(255);