Pot: require plant params and ambient temperature via setters, not the constructor

Pot.__init__ now only takes dt; set_plant_params() and set_ambient_temperature()
must be called before process(), which now raises RuntimeError if either is
missing instead of silently computing on whatever the constructor happened to
default to. set_ambient_temperature() only seeds the plant's starting
temperature the first time it's called, so changing ambient mid-run still
can't clobber an in-progress simulated temperature. Updates all callers
(server/brewpi.py, TempController(Smith)'s two internal models,
SudForecastEstimator, and the plant/pid/sud demo scripts) to call the setters
right after construction.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DkkuG48uHFCGKe6dPSERFk

scripts/demos/pid/demo_temp_controller.py

@@ -45,7 +45,9 @@ if __name__ == '__main__':
 	heatrate_soll = 1.25
 	ctrl = TempController(dt, ctrl_params)
 	ctrl.set_enabled(True)
-	plant = Pot(dt, plant_params, theta_amb)
+	plant = Pot(dt)
+	plant.set_plant_params(plant_params)
+	plant.set_ambient_temperature(theta_amb)
 	_y = np.empty(0)
 	_fb = np.empty(0)
 	_t = np.empty(0)