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/core/database.py

@@ -279,6 +279,7 @@ async def init_db():
         print_log,
         print_queue,
         printer,
+        printer_ha_sensor,
         printer_sensor_history,
         project,
         project_bom,
@@ -3996,6 +3997,18 @@ async def run_migrations(conn):
     )
     await _migrate_backfill_variant_groups(conn)
 
+    # Migration: Home Assistant sensor alerts (#1148). The printer_ha_sensors
+    # table itself is new, so create_all() builds it; only the provider opt-in
+    # column needs adding to existing databases.
+    #
+    # DEFAULT FALSE, not DEFAULT 0: Postgres will not take an integer default
+    # for a boolean column, and _safe_execute swallows the DatatypeMismatchError
+    # — so the older "BOOLEAN DEFAULT 0" migrations above quietly do nothing on
+    # Postgres and only work there because create_all() builds the column on a
+    # fresh install. SQLite has understood FALSE since 3.23, so this spelling
+    # is the one that actually applies on both.
+    await _safe_execute(conn, "ALTER TABLE notification_providers ADD COLUMN on_ha_sensor_alert BOOLEAN DEFAULT FALSE")
+
 
 async def _migrate_backfill_variant_groups(conn) -> None:
     """Build variant groups from the slice provenance already on disk (#671 / #2570).