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

app/api/routes_instructions.py

@@ -57,6 +57,14 @@ def update_instruction(instruction_id: str, body: UpdateInstructionRequest):
     return InstructionCreateResponse(item=item)
 
 
+@router.delete("/consumed", status_code=200)
+def clear_consumed_instructions():
+    """Delete all consumed instructions. Pending instructions are never affected."""
+    count = instruction_service.clear_consumed()
+    event_service.broadcast("history.cleared", {"count": count})
+    return {"cleared": count}
+
+
 @router.delete("/{instruction_id}", status_code=204)
 def delete_instruction(instruction_id: str):
     try: