Reporter's H2C connected over MQTT+TLS but every status field stayed
unknown. Root cause was layer-8: the MQTT broker is the printer, it
authenticates on the access code and SUBACKs any topic string, so a
wrong or mis-cased serial connects fine and silently receives nothing
(the report topic device/<serial>/report is case-sensitive; Bambu
serials are uppercase). Bambuddy stored and used the serial verbatim.
- schemas/printer.py: field_validator strip()+upper()s serial_number on
create, rejects blank-after-strip. The subscribed topic now always
matches the printer's correctly-cased one.
- bambu_mqtt.py: count report-topic messages per connection; when a
stale reconnect fires with zero reports received, log a one-shot
hint pointing at the serial number instead of looping silently.
