modbus: add explicit cascade delete (device + readings + toggles + HA cleanup)

A disabled device with historical readings still cannot be deleted by default
(409 guard stays, to prevent accidental data loss). A new explicit opt-in lets
the user remove it for real:

- DELETE /api/modbus/devices/{uuid}?cascade=true removes the device's readings
  and exposed_entity_toggle rows, then the device, in one transaction (FK order:
  readings before device). Best-effort clears the HA discovery configs first
  (MQTT not connected -> no-op, never blocks the delete). Default (no cascade)
  unchanged. Returns 200 with deletion counts.
- Frontend: a 'Force Delete (all data)' button appears only after the 409, with
  a red irreversible warning; cascade is sent only on that explicit action.
- Also corrected a stale comment: FK RESTRICT IS enforced at runtime now.

frontend/src/energy/hooks.test.tsx

@@ -173,7 +173,7 @@ describe('useUpdateDevice', () => {
 describe('useDeleteDevice', () => {
   beforeEach(() => vi.clearAllMocks())
 
-  it('calls DELETE /api/modbus/devices/{uuid}', async () => {
+  it('calls DELETE /api/modbus/devices/{uuid} without query param when cascade omitted', async () => {
     mockDelete.mockResolvedValue({ data: null })
     mockGet.mockResolvedValue({ data: { items: [], total: 0 } })
 
@@ -182,13 +182,32 @@ describe('useDeleteDevice', () => {
     const { result } = renderHook(() => useDeleteDevice(), { wrapper: Wrapper })
 
     await act(async () => {
-      await result.current.mutateAsync('test-uuid-1')
+      await result.current.mutateAsync({ uuid: 'test-uuid-1' })
     })
 
     expect(mockDelete).toHaveBeenCalledWith('/api/modbus/devices/{uuid}', {
       params: { path: { uuid: 'test-uuid-1' } },
     })
   })
+
+  it('calls DELETE /api/modbus/devices/{uuid} with cascade=true in query when cascade=true', async () => {
+    mockDelete.mockResolvedValue({
+      data: { deleted: true, readings_deleted: 5, toggles_deleted: 2 },
+    })
+    mockGet.mockResolvedValue({ data: { items: [], total: 0 } })
+
+    const { Wrapper } = makeWrapper()
+    const { useDeleteDevice } = await import('./hooks')
+    const { result } = renderHook(() => useDeleteDevice(), { wrapper: Wrapper })
+
+    await act(async () => {
+      await result.current.mutateAsync({ uuid: 'test-uuid-1', cascade: true })
+    })
+
+    expect(mockDelete).toHaveBeenCalledWith('/api/modbus/devices/{uuid}', {
+      params: { path: { uuid: 'test-uuid-1' }, query: { cascade: true } },
+    })
+  })
 })
 
 describe('useTestReadDevice', () => {

frontend/src/energy/hooks.ts

@@ -115,12 +115,21 @@ export function useUpdateDevice() {
 // Mutation: delete device
 // ---------------------------------------------------------------------------
 
+export interface DeleteDeviceParams {
+  uuid: string
+  /** When true, perform a cascade delete (removes readings + expose toggles). */
+  cascade?: boolean
+}
+
 export function useDeleteDevice() {
   const qc = useQueryClient()
   return useMutation({
-    mutationFn: (uuid: string) =>
+    mutationFn: ({ uuid, cascade }: DeleteDeviceParams) =>
       apiClient.DELETE('/api/modbus/devices/{uuid}', {
-        params: { path: { uuid } },
+        params: {
+          path: { uuid },
+          ...(cascade ? { query: { cascade: true } } : {}),
+        },
       }),
     onSuccess: () => qc.invalidateQueries({ queryKey: ['modbus-devices'] }),
   })