refactor(R2d.125): carve internal/api/aibatthealth subpackage

Move ai_battery_health_handler.go + test (LLM battery-health forecast narrative) into aibatthealth subpkg, rename AIBatteryHealthHandler -> Handler. Distinct from non-AI internal/api/battery subpkg. Router uses aibatthealth.NewHandler; AIHandlers.BatteryHealth field accepts it.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: atulmgupta

internal/api/aibatthealth/doc.go

@@ -0,0 +1,8 @@
+// Package aibatthealth serves the AI battery-health forecast narrative endpoint.
+//
+// It owns the LLM narration handler and the read-only forecaster adapter used by
+// query_battery_health_forecast while preserving the deterministic battery
+// degradation endpoints as the canonical baseline.
+//
+// Layer: handler
+package aibatthealth