feat: add Clear History button to delete all consumed instructions

- Backend: instruction_service.clear_consumed() bulk-deletes consumed rows
- Backend: DELETE /api/instructions/consumed route (preserves pending)
- Frontend: Clear button in consumed panel header (hidden when empty)
- Frontend: SSE handler for history.cleared event - instant UI update
- Frontend: api.clearConsumed() fetch wrapper

static/js/api.js

@@ -35,6 +35,7 @@ export const api = {
   createInstruction: (content)     => request('POST', '/api/instructions', { content }),
   updateInstruction: (id, content) => request('PATCH', `/api/instructions/${id}`, { content }),
   deleteInstruction: (id)          => request('DELETE', `/api/instructions/${id}`),
+  clearConsumed: ()                => request('DELETE', '/api/instructions/consumed'),
 
   // Config
   getConfig: ()                   => request('GET', '/api/config'),