FU11: anchor cumulative cost at recording start; add daily-reset cost entities; prefill add-version form

- MQTT import_cost_total / export_revenue_total now anchor fixed-fee / tax-credit
  accrual at max(contract start, first recorded period), so standing charges for
  untracked pre-recording days are no longer folded into the running total.
- Add energy.import_cost_today / energy.export_revenue_today (monetary,
  total_increasing) that reset at server-local midnight, for per-day cost
  aggregation in Home Assistant alongside the running totals.
- ContractForm add-version mode prefills the contract's open version values, so
  only the fields that actually change need editing.

tests/test_expose_catalog.py

@@ -205,9 +205,10 @@ def test_register_provider_direct_call():
 def test_build_catalog_empty_with_no_devices(expose_db):
     """build_catalog with no enabled modbus devices must contain no modbus entities.
 
-    The energy_cost provider is always registered and always produces its 4 entities
-    regardless of device state, so the catalog will not be empty.  This test checks
-    that no *modbus* entities are present when there are no enabled modbus devices.
+    The energy_cost provider is always registered and always produces its 6 entities
+    (4 original + 2 daily) regardless of device state, so the catalog will not be empty.
+    This test checks that no *modbus* entities are present when there are no enabled
+    modbus devices.
     """
     from app.integrations.expose import build_catalog
 
@@ -215,16 +216,16 @@ def test_build_catalog_empty_with_no_devices(expose_db):
         catalog = build_catalog(session)
 
     # The modbus provider finds no enabled devices → no modbus entities.
-    # The energy_cost provider always produces 4 entities, so the catalog is non-empty.
+    # The energy_cost provider always produces 6 entities, so the catalog is non-empty.
     modbus_entities = [e for e in catalog if e.entity.key.startswith("modbus.")]
     assert modbus_entities == [], (
         "Expected no modbus entities when no modbus devices are enabled"
     )
 
-    # The 4 energy_cost entities should always be present.
+    # The 6 energy_cost entities should always be present (4 original + 2 daily).
     energy_keys = {e.entity.key for e in catalog if e.entity.key.startswith("energy.")}
-    assert len(energy_keys) == 4, (
-        f"Expected exactly 4 energy_cost entities, got {energy_keys!r}"
+    assert len(energy_keys) == 6, (
+        f"Expected exactly 6 energy_cost entities (4 original + 2 daily), got {energy_keys!r}"
     )