Replace inline hex literals throughout hub.py with named constants in
const.py. Improves readability and protects against typos like the
limits_step_down bug that previously sent CMD_LIMITS_STEP_UP.

Also adds set_speed default case to prevent UnboundLocalError when
_blind_speed is unset or unrecognised.

New diagnostic sensors:
- ConnectionStatusSensor: Bluetooth connectivity state (binary)
- LockStatusSensor: blind busy/idle state (binary, LOCK device class)
- TuissModelSensor: blind hardware model
- TuissBlindSpeedSensor: current speed setting (gated by model support)
- TuissLastBatteryCheckSensor: timezone-aware timestamp of last battery check
- TuissBatteryCheckIntervalSensor: configured check interval
- TuissTraversalSpeedSensor: measured traversal speed (% per second)
- TuissLastConnectionErrorSensor: most recent connection error

Quality fixes:
- Seed _blind_speed/_battery_check_days/_restart_attempts from options at
  boot. update_listener only fires on option CHANGES, leaving values None
  until first edit, which makes set_speed() raise UnboundLocalError and
  diagnostic sensors read None.
- Read live model from blind object in options flow. Device registry's
  model field is set at registration and may hold the BLE advertisement
  name ("Tuiss Smartview") rather than hardware model ("TS2600"),
  hiding model-specific options like speed control.
- Switch _last_battery_check to dt_util.now() for timezone awareness.
- Call publish_updates() after option changes and lock state changes so
  diagnostic sensors reflect the new value without waiting for a poll.

The test was failing because str(exc.value) on the HomeAssistantError
raised by async_add_timer triggers HA's translation lookup, which
calls async_get_hass() and fails when invoked from a synchronous
test assertion outside a running event loop:

    HomeAssistantError: async_get_hass called from the wrong thread

The integration code is correct — the regression is in the test's
choice to stringify the exception. Switch the assertion to read the
translation_key attribute directly (HA stores translation kwargs as
plain attributes on the exception instance) so we never invoke
__str__ and never touch the translation machinery.

Full suite now: 83 passed, 0 failed.

- Drop unused UnitOfTime import in sensor.py
- Use CMD_TIMESTAMP_BASE constant in hub.send_timestamp() for consistency
  with the rest of the BLE-command refactor