Add Fresh Harvest integration scaffold

Implements the freshharvest.com two-step login handshake, config flow,
6-hour polling coordinator, and four delivery sensors. Portal page parsing
is not implemented; async_get_next_delivery raises until the account HTML
is mapped against a signed-in session.

custom_components/freshharvest/const.py

@@ -0,0 +1,12 @@
+"""Constants for the Fresh Harvest integration."""
+
+from __future__ import annotations
+
+from datetime import timedelta
+
+DOMAIN = "freshharvest"
+
+# The portal is a small business's site, not an API. Poll gently: delivery
+# schedules change on the order of days, and the customization cutoff is the
+# only time-sensitive value.
+UPDATE_INTERVAL = timedelta(hours=6)