docs: note config.json's by-id serial paths and the .bak cleanup

config.json isn't tracked in git, so this records what changed on the
Pi during today's connect/disconnect testing: Heater/Stirrer ports
switched from /dev/ttyUSB0//dev/ttyACM0 to stable /dev/serial/by-id/...
paths after a replug renumbered both devices, plus a reminder to
remove the pre-edit config.json.bak-20260703212821 left on the Pi.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YaPLuRPpyjWcwhMvCvpHCL

docs/pi_deployment_notes.md

@@ -0,0 +1,32 @@
+# brewpi Pi deployment notes
+
+`config.json` is gitignored (`.gitignore:6`) - it's generated per-deployment
+from `config-real.json.tpl`/`config-sim.json.tpl`, not tracked in git, so
+notes about its actual on-Pi content have to live here instead.
+
+## Heater/Stirrer `port` should use `/dev/serial/by-id/...`, not `/dev/ttyUSB0`/`/dev/ttyACM0`
+
+Found 2026-07-03: unplugging the Hendi mid-brew (to test the connect/
+disconnect feature, and the crash-recovery fix that followed) caused both
+USB-serial adapters to renumber on replug - `/dev/ttyUSB0` became
+`/dev/ttyUSB1`, `/dev/ttyACM0` became `/dev/ttyACM1`. `config.json` still
+pointed at the old raw device paths, so a plain reconnect kept failing
+(wrong port, not a code bug) until `config.json` was updated to the stable
+`/dev/serial/by-id/...` symlinks udev already provides on this Pi:
+
+```
+Heater.port:  /dev/serial/by-id/usb-JDI_Hendi-Control_AM3VLPVE-if00-port0
+Stirrer.port: /dev/serial/by-id/usb-Pololu_Corporation_Pololu_Simple_High-Power_Motor_Controller_18v15_53FF-6F06-7277-5049-3950-0767-if00
+```
+
+Since raw `/dev/ttyUSB*`/`/dev/ttyACM*` numbering isn't stable across
+replugs (and a real unplug is exactly the scenario the connect/disconnect
+feature exists to handle gracefully), `config.json` should keep using the
+by-id paths above rather than reverting to `/dev/ttyUSB0`/`/dev/ttyACM0`.
+
+## Cleanup: `config.json.bak-20260703212821`
+
+A timestamped backup (`config.json.bak-20260703212821`) was left next to
+`config.json` on the Pi before the by-id edit above, in case it needed to
+be reverted. It wasn't needed - safe to delete once this note has been
+read: `rm /home/jens/brewpi/config.json.bak-20260703212821`.