Make timezone required in BatcontrolConfig (no default)

The dummy config documents timezone as "not optional" and core logic
uses it for scheduling/price-window alignment. Silently defaulting to
'Europe/Berlin' masks misconfigured setups. Now fails fast at config
load with a clear validation error when timezone is absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: MaStr

src/batcontrol/config_model.py

@@ -269,7 +269,7 @@ class BatcontrolConfig(BaseModel):
     """
     model_config = ConfigDict(extra='allow')
 
-    timezone: str = 'Europe/Berlin'
+    timezone: str
     time_resolution_minutes: int = 60
     loglevel: str = 'info'
     logfile_enabled: bool = True