perf: import heavy optional dependencies on first use (#1377)

A GENETIC server with external (import) providers and EOSdash disabled
loads several large packages at start that it never uses, which costs
a noticeable part of the memory on 512 MB boards:

- matplotlib via the GENETIC/GENETIC0 PDF report modules (server/eos.py)
- statsmodels via the Holt-Winters fallback in prediction/priceabc.py
- pvlib via the weather base class and the pvlib-based providers
- bs4/lxml via the ClearOutside provider
- scipy via the self-consumption interpolator singletons, which were
  loaded at import time
- the GENETIC0 optimizer, imported by core/ems.py
- tzfpy, whose polygon data the computed general.timezone loads

Import these where they are used. The interpolator tables are loaded
on first use (thread-safe). Nothing changes in what is computed.

general.timezone is computed from latitude/longitude and cannot be set.
Add general.timezone_override (IANA name, validated with pendulum):
when set, general.timezone returns it and the lookup is skipped. When
unset (default) the lookup is unchanged.

Tests that patched the moved names now patch them at their source.
A new test starts the server module in a subprocess and checks that
none of these modules is loaded.

Co-authored-by: Christin <christin@BigBrother.bikini.bottom.zone>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_pvforecastpvlib.py

@@ -1308,8 +1308,8 @@ class TestCalculation:
         assert isinstance(result, pd.DataFrame)
         assert result.empty
 
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.ModelChain")
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.Location")
+    @patch("pvlib.modelchain.ModelChain")
+    @patch("pvlib.location.Location")
     def test_location_created(
         self,
         mock_location,
@@ -1352,7 +1352,7 @@ class TestCalculation:
 
         mock_location.assert_called_once()
 
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.ModelChain")
+    @patch("pvlib.modelchain.ModelChain")
     def test_modelchain_run_called(
         self,
         mock_modelchain,
@@ -1395,7 +1395,7 @@ class TestCalculation:
 
         mc.run_model.assert_called_once_with(weather)
 
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.ModelChain")
+    @patch("pvlib.modelchain.ModelChain")
     def test_negative_power_clipped(
         self,
         mock_modelchain,
@@ -1444,7 +1444,7 @@ class TestCalculation:
         assert (result["pv_dc_power"] >= 0).all()
         assert (result["ac_power"] >= 0).all()
 
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.ModelChain")
+    @patch("pvlib.modelchain.ModelChain")
     def test_dataframe_columns(
         self,
         mock_modelchain,
@@ -1489,7 +1489,7 @@ class TestCalculation:
         assert "pv_dc_power" in result.columns
         assert "ac_power" in result.columns
 
-    @patch("akkudoktoreos.prediction.pvforecastpvlib.ModelChain")
+    @patch("pvlib.modelchain.ModelChain")
     def test_result_index_preserved(
         self,
         mock_modelchain,