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/pages/EnergyPage.test.tsx

@@ -288,6 +288,7 @@ describe('EnergyPage — delete device', () => {
 
     await waitFor(() => expect(screen.getByTestId(`device-delete-${DEVICE.uuid}`)).toBeInTheDocument())
     fireEvent.click(screen.getByTestId(`device-delete-${DEVICE.uuid}`))
+    // On first open (no 409 yet), the regular confirm button is visible.
     await waitFor(() => expect(screen.getByTestId('device-delete-confirm')).toBeInTheDocument())
 
     fireEvent.click(screen.getByTestId('device-delete-confirm'))
@@ -295,6 +296,38 @@ describe('EnergyPage — delete device', () => {
     await waitFor(() => {
       expect(screen.getByTestId('device-delete-409-hint')).toBeInTheDocument()
     })
+    // After 409, the force-delete button appears instead of the normal confirm.
+    expect(screen.getByTestId('device-delete-force')).toBeInTheDocument()
+    expect(screen.queryByTestId('device-delete-confirm')).not.toBeInTheDocument()
+  })
+
+  it('force-delete button triggers cascade delete after 409', async () => {
+    const { ApiError } = await import('../api/client')
+    // First call → 409, second call (cascade) → success
+    mockDelete
+      .mockRejectedValueOnce(new ApiError(409, { detail: 'device has readings' }))
+      .mockResolvedValueOnce({ data: { deleted: true, readings_deleted: 3, toggles_deleted: 1 } })
+
+    renderEnergy()
+
+    await waitFor(() => expect(screen.getByTestId(`device-delete-${DEVICE.uuid}`)).toBeInTheDocument())
+    fireEvent.click(screen.getByTestId(`device-delete-${DEVICE.uuid}`))
+    await waitFor(() => expect(screen.getByTestId('device-delete-confirm')).toBeInTheDocument())
+
+    // First attempt → 409
+    fireEvent.click(screen.getByTestId('device-delete-confirm'))
+
+    // Force-delete button appears
+    await waitFor(() => expect(screen.getByTestId('device-delete-force')).toBeInTheDocument())
+
+    // Click force-delete → should call DELETE with cascade=true
+    fireEvent.click(screen.getByTestId('device-delete-force'))
+
+    await waitFor(() => {
+      expect(mockDelete).toHaveBeenCalledWith('/api/modbus/devices/{uuid}', {
+        params: { path: { uuid: DEVICE.uuid }, query: { cascade: true } },
+      })
+    })
   })
 })
 

frontend/src/pages/EnergyPage.tsx

@@ -57,13 +57,22 @@ import { formatLocalDateTime } from '../utils/datetime'
 interface ConfirmDeleteProps {
   device: ModbusDevice
   onConfirm: () => void
+  /** Called when the user explicitly opts into cascade (force) deletion. */
+  onForceDelete: () => void
   onCancel: () => void
   loading: boolean
   /** Set when the delete failed with 409. */
   has409Error: boolean
 }
 
-function ConfirmDeleteModal({ device, onConfirm, onCancel, loading, has409Error }: ConfirmDeleteProps) {
+function ConfirmDeleteModal({
+  device,
+  onConfirm,
+  onForceDelete,
+  onCancel,
+  loading,
+  has409Error,
+}: ConfirmDeleteProps) {
   return (
     <Modal
       opened
@@ -78,24 +87,43 @@ function ConfirmDeleteModal({ device, onConfirm, onCancel, loading, has409Error
         </Text>
 
         {has409Error && (
-          <Alert color="orange" data-testid="device-delete-409-hint">
-            This device has existing readings and cannot be deleted. Consider{' '}
-            <strong>disabling</strong> it instead (click Edit → uncheck Enabled).
-          </Alert>
+          <>
+            <Alert color="orange" data-testid="device-delete-409-hint">
+              This device has existing readings and cannot be deleted. Consider{' '}
+              <strong>disabling</strong> it instead (click Edit → uncheck Enabled).
+            </Alert>
+            <Alert color="red" variant="light" data-testid="device-delete-force-warning">
+              Alternatively, you can <strong>permanently delete</strong> the device together with{' '}
+              all its historical readings and expose settings. This action{' '}
+              <strong>cannot be undone</strong>.
+            </Alert>
+          </>
         )}
 
         <Group justify="flex-end" gap="sm">
           <Button variant="default" onClick={onCancel} data-testid="device-delete-cancel">
             Cancel
           </Button>
-          <Button
-            color="red"
-            loading={loading}
-            onClick={onConfirm}
-            data-testid="device-delete-confirm"
-          >
-            Delete
-          </Button>
+          {!has409Error && (
+            <Button
+              color="red"
+              loading={loading}
+              onClick={onConfirm}
+              data-testid="device-delete-confirm"
+            >
+              Delete
+            </Button>
+          )}
+          {has409Error && (
+            <Button
+              color="red"
+              loading={loading}
+              onClick={onForceDelete}
+              data-testid="device-delete-force"
+            >
+              Force Delete (all data)
+            </Button>
+          )}
         </Group>
       </Stack>
     </Modal>
@@ -532,7 +560,7 @@ function DevicesTab() {
     if (!deleteDevice) return
     setDelete409(false)
     try {
-      await deleteMutation.mutateAsync(deleteDevice.uuid)
+      await deleteMutation.mutateAsync({ uuid: deleteDevice.uuid })
       closeDelete()
     } catch (err) {
       if (err instanceof ApiError && err.status === 409) {
@@ -542,6 +570,17 @@ function DevicesTab() {
     }
   }
 
+  async function handleForceDelete() {
+    if (!deleteDevice) return
+    try {
+      await deleteMutation.mutateAsync({ uuid: deleteDevice.uuid, cascade: true })
+      closeDelete()
+    } catch {
+      // If cascade delete also fails for some reason, keep the modal open.
+      // (Should be very rare — only if the device was concurrently deleted.)
+    }
+  }
+
   if (devicesQuery.isLoading) {
     return (
       <Center pt="xl" data-testid="energy-loading">
@@ -600,6 +639,7 @@ function DevicesTab() {
         <ConfirmDeleteModal
           device={deleteDevice}
           onConfirm={handleDeleteConfirm}
+          onForceDelete={handleForceDelete}
           onCancel={closeDelete}
           loading={deleteMutation.isPending}
           has409Error={delete409}