feat(scheduler): Immutable schedules and HTTP-only pipeline

## Changes
- **Migration 089**: Add is_immutable and method columns to task_schedules
  - Per-state product_discovery schedules (4h default)
  - Store discovery weekly (168h)
  - All schedules use HTTP transport (Puppeteer/browser)
- **Task Scheduler**: HTTP-only product discovery with per-state scheduling
  - Each state has its own immutable schedule
  - Schedules can be edited (interval/priority) but not deleted
- **TasksDashboard UI**: Full immutability support
  - Lock icon for immutable schedules
  - State and Method columns in schedules table
  - Disabled delete for immutable, restricted edit fields
- **Store Discovery HTTP**: Auto-queue product_discovery for new stores
- **Migration 088**: Discovery payloads storage schema

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/tasks/handlers/index.ts

@@ -13,6 +13,7 @@ export { handleProductDiscoveryHttp } from './product-discovery-http';
 export { handlePayloadFetch as handlePayloadFetchCurl } from './payload-fetch-curl';
 export { handleProductRefresh } from './product-refresh';
 export { handleStoreDiscovery } from './store-discovery';
+export { handleStoreDiscoveryHttp } from './store-discovery-http';
 export { handleEntryPointDiscovery } from './entry-point-discovery';
 export { handleAnalyticsRefresh } from './analytics-refresh';
 export { handleWhoami } from './whoami';