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_optimization_pdf_api.py

@@ -9,6 +9,7 @@ import pytest
 from fastapi.testclient import TestClient
 from pypdf import PdfReader
 
+from akkudoktoreos.optimization.genetic import geneticvisualize
 from akkudoktoreos.optimization.genetic.geneticsolution import GeneticSolution
 from akkudoktoreos.server import eos
 from akkudoktoreos.utils.datetimeutil import to_datetime
@@ -70,7 +71,8 @@ async def test_render_is_offloaded_and_uses_an_owned_copy(config_eos, monkeypatc
         solution.ac_charge[0] = 0.123
         return b"%PDF-isolated-render"
 
-    monkeypatch.setattr(eos, "genetic_prepare_visualize", render)
+    # The PDF stack is imported on first use, so patch it at its source.
+    monkeypatch.setattr(geneticvisualize, "genetic_prepare_visualize", render)
     response = await eos.fastapi_energy_management_optimization_solution_genetic_pdf_get()
     assert response.body == b"%PDF-isolated-render"
     assert captured[0][0] != caller_thread