Leave out unusable rooms instead of pausing; add keep-last-value option

A single stale, unavailable, missing or invalid room sensor paused
learning and forecasts for every room. That is not acceptable once the
MPC may control heating.

Now such a room is left out: the weighted temperature, learning and the
optimizer use the remaining rooms with their weights re-normalized, and
a room learns only across poll pairs where it was usable at both ends.
Readiness considers only the rooms used. Everything pauses only when no
room is usable or a shared input (relay, thermostat, outdoor
temperature) is missing.

A new setup/Configure checkbox, room_stale_use_last_value (default off),
keeps using a stale room sensor's last value instead, for sensors that
report only on change. It never applies to unavailable or invalid
sensors, or to a sensor silent for more than 24 hours.

Status adds excluded_rooms, weight_coverage and a per-room "used" flag.
English and Romanian labels and help text added.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_data_quality.py

@@ -48,5 +48,7 @@ class FreshnessTests(unittest.TestCase):
             data = json.loads((ROOT / name).read_text())
             for section, step in (('config', 'user'), ('options', 'init')):
                 labels = data[section]['step'][step]['data']
-                for key in ('room_max_age_minutes', 'weather_max_age_minutes'):
+                for key in ('room_max_age_minutes', 'weather_max_age_minutes', 'room_stale_use_last_value'):
                     self.assertTrue(labels[key])
+                help_text = data[section]['step'][step]['data_description']
+                self.assertIn('24', help_text['room_stale_use_last_value'])