pyproject.toml: Enable mypy.warn_unreachable

Add warn_unreachable = true to the mypy configuration. This detects
statements that mypy proves are unreachable, such as code after
an unconditional return or assertions that can never be true.

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

conf/templates/pyproject.toml

@@ -10,6 +10,8 @@
 
   warn_unused_ignores = true
 
+  warn_unreachable = true
+
   enable_error_code = [
     "explicit-override",
     "ignore-without-code",