Fix A1 Mini "unknown" status from non-UTF-8 MQTT payload (#549)

  Some firmware versions send MQTT payloads with non-UTF-8 bytes.
  UnicodeDecodeError was uncaught, silently dropping the entire message
  and leaving printer status as "unknown" with 0°C temps and no AMS.

  Fall back to decode(errors="replace") to keep JSON parseable.

CHANGELOG.md

@@ -18,6 +18,7 @@ All notable changes to Bambuddy will be documented in this file.
 - **Queue Badge Shows on Incompatible Printers** ([#486](https://github.com/maziggy/bambuddy/issues/486)) — The purple queue counter badge in the printer card header showed on all printers of the same model when a job was scheduled for "any [model]", even if the printer didn't have the matching filament color loaded. The `PrinterQueueWidget` (which shows "Clear Plate & Start") already filtered by filament type and color, but the badge count used the raw unfiltered queue length. Now applies the same filament compatibility filter to the badge count.
 - **SpoolBuddy Daemon Can't Find Hardware Drivers** — The daemon's `nfc_reader.py` and `scale_reader.py` import `read_tag` and `scale_diag` as bare modules, but these files live in `spoolbuddy/scripts/` which isn't on Python's module search path. The systemd service sets `WorkingDirectory` to `spoolbuddy/` and runs `python -m daemon.main`, so only the `spoolbuddy/` and `daemon/` directories are on `sys.path`. Added `scripts/` to `sys.path` at daemon startup, resolved relative to the module file so it works regardless of install path. Also moved the `read_tag` import inside `NFCReader.__init__`'s try/except block — it was previously outside, so a missing module crashed the entire daemon instead of gracefully skipping NFC polling. Demoted hardware-not-available log messages from ERROR to INFO since missing modules are expected when hardware isn't connected.
 - **SpoolBuddy Scale Tare & Calibration Not Applied** — The SpoolBuddy scale tare and calibrate buttons on the Settings page queued commands but never executed them. Five bugs in the chain: (1) the daemon received the `tare` command via heartbeat but never called `scale.tare()` — a comment said "need cross-task communication" but the ScaleReader was already available in the shared dict; (2) no API endpoint existed for the daemon to report the new tare offset back to the backend database, so tare results were lost; (3) when calibration values changed in heartbeat responses, the daemon updated its config object but never called `scale.update_calibration()`, so the ScaleReader kept using its initial values forever; (4) the heartbeat response that delivered the tare command still contained pre-tare calibration values, which immediately overwrote the new tare offset back to zero; (5) the `set-factor` endpoint computed `calibration_factor` using the DB `tare_offset`, which could be stale or zero if the tare hadn't persisted yet — producing a wildly wrong factor (e.g., 5000g displayed with empty scale). Added a `POST /devices/{device_id}/calibration/set-tare` endpoint and `update_tare()` API client method. The heartbeat loop now executes `scale.tare()` when the tare command is received, persists the result via the new endpoint, propagates calibration changes to the ScaleReader instance, and skips calibration sync on the heartbeat cycle that delivers a tare command. The calibration flow now captures the raw ADC at tare time and sends it alongside the loaded-weight ADC in step 2, so the factor is computed from the actual tare reference rather than the DB value — making calibration self-contained and independent of the tare persistence round-trip. The calibration weight input uses a compact touch-friendly numpad since the RPi kiosk has no physical keyboard.
+- **A1 Mini Shows "Unknown" Status After MQTT Payload Decode Failure** ([#549](https://github.com/maziggy/bambuddy/issues/549)) — Some printer firmware versions (observed on A1 Mini 01.07.02.00) occasionally send MQTT payloads containing non-UTF-8 bytes. The `_on_message` handler called `msg.payload.decode()` (strict UTF-8), and the resulting `UnicodeDecodeError` was not caught — only `json.JSONDecodeError` was handled. The entire message was silently dropped, causing printer status to show "unknown", temperatures to read 0°C, and AMS data to disappear. Now catches `UnicodeDecodeError` and falls back to `decode(errors="replace")`, which substitutes invalid bytes with U+FFFD while keeping the JSON structure intact. Logs a warning for diagnostics.
 - **SpoolBuddy NFC Reader Fails to Detect Tags** — The PN5180 NFC reader had two polling issues. First, each `activate_type_a()` call that returned `None` (no tag) corrupted the PN5180 transceive state — subsequent calls silently failed even when a tag was physically present, making it impossible to detect tags placed after startup (only tags already on the reader during init were detected). Fixed by performing a full hardware reset (RST pin toggle + RF re-init, ~240ms) before every idle poll, giving a ~1.8 Hz effective poll rate. Second, after a successful SELECT the card stayed in ACTIVE state and ignored subsequent WUPA/REQA, causing false "tag removed" events after ~1 second. Fixed with a light RF off/on cycle (13ms) before each poll when a tag is present, resetting the card to IDLE for re-selection. Also added error-based auto-recovery (full hardware reset after 10 consecutive poll exceptions), periodic status logging every 60 seconds, and accurate heartbeat reporting of NFC/scale health.
 
 ### Improved

backend/app/services/bambu_mqtt.py

@@ -442,7 +442,19 @@ class BambuMQTTClient:
 
     def _on_message(self, client, userdata, msg):
         try:
-            payload = json.loads(msg.payload.decode())
+            try:
+                raw = msg.payload.decode()
+            except UnicodeDecodeError:
+                # Some firmware versions (e.g. A1 Mini 01.07.02.00) send payloads
+                # with non-UTF-8 bytes. Replace invalid bytes to keep JSON parseable.
+                raw = msg.payload.decode(errors="replace")
+                logger.warning(
+                    "[%s] MQTT payload contained non-UTF-8 bytes (topic=%s, len=%d)",
+                    self.serial_number,
+                    msg.topic,
+                    len(msg.payload),
+                )
+            payload = json.loads(raw)
             # Track last message time - receiving a message proves we're connected
             self._last_message_time = time.time()
             self.state.connected = True