feat(usage-tracker): add diagnostic logging for slot-to-tray mapping investigation

Usage tracking works on X1E but fails on H2D Pro — the wrong tray
is identified, attributing usage to the wrong spool. The root cause
is unknown because we don't know what MQTT mapping data each printer
model reports.

Adds INFO-level logging at print start and completion:
- state.raw_data["mapping"] (unknown format, preserved but never parsed)
- tray_now, last_loaded_tray values
- All raw_data keys containing "map" (discovery)
- Per-AMS-tray summary: tray_type, tray_color, tray_now, tray_tar

This enables comparing MQTT behavior across printer models without
requiring DEBUG mode, so testers can send normal logs.

backend/tests/unit/test_usage_tracker.py

@@ -235,6 +235,8 @@ class TestOnPrintComplete:
         printer_manager = MagicMock()
         printer_manager.get_status.return_value = SimpleNamespace(
             raw_data={"ams": [{"id": 0, "tray": [{"id": 0, "remain": 70}]}]},
+            tray_now=0,
+            last_loaded_tray=-1,
         )
 
         # db returns assignment then spool