M6-T07: add metering billing engine + period job + summary + recompute

- app/services/energy_cost.py: register_at (register values at a UTC boundary),
  compute_period (register deltas x active-version strategy, snapshot price +
  contract_version_id, immutable unless overwrite), compute_closed_periods
  (bounded lookback), recompute_range (explicit overwrite), summarize
  (sum net + standing fees month/30 - heffingskorting year/365).
- Missing tibber price -> skip; missing reading -> degraded (zeroed amounts,
  null version), consistently on both create and recompute paths.
- main.py: 1-minute energy-cost job (existing jobs/MQTT untouched). Tests added.

app/main.py

@@ -33,6 +33,7 @@ from app.services.public_ip import check_public_ipv4_and_notify
 from app.services.modbus_poll import poll_all_enabled_devices, BASE_POLL_TICK_SECONDS
 from app.services.ha_discovery import publish_discovery, publish_states
 from app.services.tibber_prices import refresh_prices
+from app.services.energy_cost import compute_closed_periods
 from scripts.app_db_adopt import AppDatabaseAdoptionError, validate_app_runtime_db
 
 logger = logging.getLogger(__name__)
@@ -92,6 +93,28 @@ def _run_scheduled_tibber_refresh() -> None:
         session.close()
 
 
+def _run_scheduled_energy_cost() -> None:
+    """Scheduled job: compute billing records for all uncalculated closed 15-minute periods.
+
+    Runs every minute so that a new period is picked up within 1 minute of
+    closing.  The job is a no-op when:
+    - No active energy contract with a version covering the period exists.
+    - DSMR data has not yet arrived for the period boundaries.
+    - The period's billing record already exists and is not degraded.
+
+    Any unexpected exceptions are caught and logged so that a single failure
+    does not crash the scheduler or affect the other background jobs.
+    """
+    session_local = get_session_local()
+    session: Session = session_local()
+    try:
+        compute_closed_periods(session)
+    except Exception:
+        logger.exception("_run_scheduled_energy_cost: unexpected error")
+    finally:
+        session.close()
+
+
 def _run_scheduled_ha_state_publish() -> None:
     """Periodic job: publish discovery configs + state + availability for all enabled exposed entities.
 
@@ -188,6 +211,17 @@ async def lifespan(_: FastAPI):
         max_instances=1,
         coalesce=True,
     )
+    # Energy cost billing: compute uncalculated closed 15-minute periods every minute.
+    # The job is a no-op when no active contract or DSMR data is present, so it is
+    # safe to register unconditionally.
+    scheduler.add_job(
+        _run_scheduled_energy_cost,
+        trigger=IntervalTrigger(minutes=1),
+        id="energy-cost",
+        replace_existing=True,
+        max_instances=1,
+        coalesce=True,
+    )
     scheduler.start()
 
     # MQTT: connect using DB-merged runtime settings so broker configured via UI