Sigenergy: make Predbat's ownership of the inverter deliberate (option A for #4454)

[mgazza]

Option A of two mutually-exclusive alternatives for #4454. Only one of this and the sibling PR should merge.

What this does

Keeps Predbat as the owner of a Sigenergy inverter, but makes that a decision rather than a race.

A Sigenergy accepts one controller at a time, and VPP mode (Predbat) and the NorthBound Interface (Axle's dispatch channel) are mutually exclusive. Today _manage_vpp_registration() runs on the 5-minute poll tick and silently switches the system back to VPP, so an Axle dispatch can hold the inverter for up to 5 minutes before being displaced with no indication of what happened.

Observed live on 2026-08-06: mode went to Northbound Integration at 19:35:18 and back to VPP at 19:40:12 — exactly one poll interval — while the log only said "controls skipped until onboard is approved".

Changes

• Reclaim VPP every 60s (SIGENERGY_VPP_RECLAIM_INTERVAL) instead of every 300s, so control is never ambiguous for long. set_operating_mode() is an MQTT publish and only fires when the mode is actually wrong, so this costs nothing against the REST rate limit. The onboard_status cache write stays on the 300s cadence.
• Log the reclaim explicitly, naming the controller being displaced, instead of a generic warning.
• Stop reporting a contended system as pending_approval. Downstream UIs render that as an amber "approve this in the Sigenergy app" banner, so every Axle event told the user to approve something needing no approval. A system in NBI is fully onboarded.
• Expose contended_by on the onboard-status sensor so support can tell contention from a genuine onboarding failure.

Predbat already ingests Axle sessions as its own export windows (load_axle_slot()), so events still run — under Predbat's plan rather than Axle's dispatch.

Trade-off

This is the "Predbat always wins" position. It's self-contained and needs no config change, but it does override Axle's schedule on every event. If Axle credits delivery from its own NBI dispatch rather than from meter data, this costs the customer the payment even though the energy flowed. That question is open in #4454.

Tests

4 new tests in tests/test_sigenergy.py: reclaim-from-third-party, contention-not-pending-approval, reclaim-cadence, and the controls-skipped message. Full Sigenergy suite 72 pass / 0 fail; unit_test.py --quick passes with 0 failures. run_pre_commit clean (ruff, black, cspell).

[mgazza]

Customer-impact tracking and fleet scope: Predictive-Cloud-Ltd/predbat-saas#1591 (private). Alternative approach: #4456.

[mgazza]

Superseded by #4457.

These two were opened as mutually-exclusive alternatives — Predbat keeps the inverter, or Axle does. That framing was wrong. A code review found that this PR's faster reclaim actively defeats the axle_control option, which meant option A had to honour axle_control regardless — and once it does, it contains option B. They were complementary all along, not alternatives.

The real either/or was never in the code: it is the policy question of whether axle_control should be enabled, which #4454 now tracks separately.

#4457 combines both, plus fixes for the three review findings (the stale set_read_only_axle flag on restart, the reclaim defeating axle_control, and the contention marker being cleared before it was ever published).