M6-T08: add energy_cost expose provider + post-billing state publish

- expose.py: _energy_cost_provider yields buy/sell_price_now + import_cost_total
  /export_revenue_total (total_increasing, monetary), value_getters read from
  energy_cost_period; 2-element identifiers so the existing HA Discovery builder
  (which indexes identifiers[1]) works. Other providers/mechanism untouched.
- main.py: energy-cost job calls publish_states after computing periods (no-op
  when MQTT/Discovery off). Tests added.
Note: energy-cost sensors lack an availability heartbeat (ha_discovery mechanism
limitation, out of scope here).

app/main.py

@@ -109,6 +109,15 @@ def _run_scheduled_energy_cost() -> None:
     session: Session = session_local()
     try:
         compute_closed_periods(session)
+        # After billing periods are computed, push fresh energy-cost state values
+        # to MQTT/HA.  publish_states is internally guarded by _should_publish
+        # (MQTT disabled / not connected → no-op), so this never raises due to
+        # unconfigured MQTT and does not block the billing job.
+        try:
+            from app.services.ha_discovery import publish_states
+            publish_states(session)
+        except Exception:
+            logger.exception("_run_scheduled_energy_cost: publish_states failed (non-fatal)")
     except Exception:
         logger.exception("_run_scheduled_energy_cost: unexpected error")
     finally: