MAINT-11: start or stop the maintenance cycle per battery

maint_start and maint_abort take an optional unit. With two or more batteries
each Batteries card has Start cycle and Stop cycle buttons. One slot per site
stays: a start while a battery cycles is refused and names it (no queue). A
battery that is switched off, an unknown unit, and a stop on a battery that is
not cycling are refused with a plain message. A request with no unit is the
old call. The scheduler and the phase machine are unchanged.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bdqr7PiaHcBnzPituu2yTC

goodwe_controller/DECISIONS/MAINT-11-cycle-per-battery.md

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+# MAINT-11 - start or stop the maintenance cycle per battery
+
+**Status: for review.** Tests: `test_maint11.py`. No option added.
+
+**The fault.** At two or more batteries the page had one "start a battery cycle"
+button, and `_maint_button()` chose the battery. An operator could not cycle a
+chosen one (inverter B, new after storage, needed its first cycle).
+
+## Rule
+
+1. `maint_start` and `maint_abort` take an optional `unit`. With no unit they
+   run the old code unchanged. The Batteries screen sends the unit from a
+   per-battery Start cycle and Stop cycle button, at two or more batteries only.
+2. **A start while any battery holds the maintenance slot is refused, not
+   queued.** The message names the battery that is cycling. One slot per site
+   stays the rule, and the scheduler is untouched.
+3. Also refused, with a plain message: an unknown unit, a start on a battery
+   that is switched off (battery modes on), and a stop on a battery that is not
+   cycling. Nothing changes on a refusal.
+4. A stop on the cycling battery is today's abort: the cycle ends, then every
+   inverter goes to 0 W.
+
+## Why refuse and not queue
+
+A queued start is a cycle that begins hours later, unseen, after the operator
+has moved on. A refusal is immediate and says why. The scheduler already
+queues by waiting time, so nothing is lost.
+
+## What this does not cover
+
+- Today's bare button never refused a battery that is refusing to give power
+  (SAFETY-73); its drain waits. The per-battery start does the same.
+- A start on the battery that is already cycling is refused as "busy", where
+  the bare button restarts its drain. The bare button is unchanged.