Show Home Assistant sensors on the printer card (#1148, #448)

Binds binary_sensor and reading-carrying sensor entities to a printer and
renders their state on its card, worded by Home Assistant's device_class.
Optional per-sensor alert condition drives a notification on the transition
into the alert state and an opt-in interlock that holds queued prints while
alerting -- a hold with a readable waiting_reason, never a failure, and only
ever on a sensor that was read successfully.

Sensors get their own table rather than a wider entity pattern on SmartPlug:
get_smart_plug_by_printer would otherwise hand the card's power button a door
contact to switch.

The hold is passed to the model matcher directly rather than merged into
busy_printers: _check_auto_drying reads that set as "is currently printing"
and would put an idle-but-held printer down the mid-print drying path.

The notification_providers migration spells its default FALSE, not 0 --
Postgres rejects an integer default for a boolean and _safe_execute swallows
the error.

backend/app/models/printer.py

@@ -61,6 +61,7 @@ class Printer(Base):
     sensor_history: Mapped[list["PrinterSensorHistory"]] = relationship(
         back_populates="printer", cascade="all, delete-orphan"
     )
+    ha_sensors: Mapped[list["PrinterHASensor"]] = relationship(back_populates="printer", cascade="all, delete-orphan")
 
 
 from backend.app.models.ams_history import AMSSensorHistory  # noqa: E402
@@ -68,5 +69,6 @@ from backend.app.models.archive import PrintArchive  # noqa: E402
 from backend.app.models.kprofile_note import KProfileNote  # noqa: E402
 from backend.app.models.maintenance import PrinterMaintenance  # noqa: E402
 from backend.app.models.notification import NotificationProvider  # noqa: E402
+from backend.app.models.printer_ha_sensor import PrinterHASensor  # noqa: E402
 from backend.app.models.printer_sensor_history import PrinterSensorHistory  # noqa: E402
 from backend.app.models.smart_plug import SmartPlug  # noqa: E402