Add comprehensive test suite achieving 99% code coverage

Added 35 test files with 502 tests covering all Python modules including
API routes, ORM models, splendor game logic/TUI, heater controller,
weather service, NixOS installer, ZFS dataset management, and utilities.
Coverage improved from 11% to 99% (2540/2564 statements covered).

https://claude.ai/code/session_01SVzgLDUS1Cdc4eh1ijETTh

tests/test_stuff_thing.py

@@ -0,0 +1,17 @@
+"""Tests for python/stuff/thing.py."""
+
+from __future__ import annotations
+
+from python.stuff.thing import caculat_batry_specs
+
+
+def test_caculat_batry_specs() -> None:
+    """Test battery specs calculation."""
+    capacity, voltage = caculat_batry_specs(
+        cell_amp_hour=300,
+        cell_voltage=3.2,
+        cells_per_pack=8,
+        packs=2,
+    )
+    assert voltage == 3.2 * 8
+    assert capacity == voltage * 300 * 2