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.

app/schemas/modbus.py

@@ -152,3 +152,20 @@ class ModbusTestReadResponse(BaseModel):
     ok: bool
     payload: dict[str, Any] | None = None
     error: str | None = None
+
+
+# ---------------------------------------------------------------------------
+# Cascade-delete schema
+# ---------------------------------------------------------------------------
+
+
+class ModbusDeleteResponse(BaseModel):
+    """Response for DELETE /api/modbus/devices/{uuid}?cascade=true.
+
+    Returned only when cascade deletion succeeds (HTTP 200).  Non-cascade
+    successful deletes continue to return HTTP 204 (no body).
+    """
+
+    deleted: bool
+    readings_deleted: int
+    toggles_deleted: int