fix(dist): stop stripping .py sources; wipe payload on NSIS upgrade

Two root causes for the 'imaging extension was built for another version
of Pillow' error users hit after install:

1) cleanup_site_packages ran 'find ... -name "*.py" ! -name "__init__.py"
   -delete' with a comment claiming 'keep .pyc only' — but no compileall
   step exists. Result: the dist shipped __init__.py + .pyd only, missing
   every submodule (Image.py, ImageDraw.py, _version.py, ...). Fresh
   installs were broken; in-place upgrades produced a half-old/half-new
   site-packages. Removed the deletion entirely.

2) NSIS installer extracted over the previous install without cleaning
   python/, app/, scripts/. Upgrades left stale files (old PIL/_version.py
   next to new PIL/_imaging.pyd) which raised the Pillow ABI mismatch.
   Wipe those subtrees before File /r, preserving config.yaml at the
   install root.

installer.nsi

@@ -70,6 +70,19 @@ Section "!Core (required)" SecCore
 
     SetOutPath "$INSTDIR"
 
+    ; Wipe previous payload before extracting so stale files from an older
+    ; version cannot survive an upgrade. Without this, in-place upgrades
+    ; produce a half-old/half-new site-packages — e.g. an old PIL/_version.py
+    ; alongside a new PIL/_imaging.pyd, which raises "_imaging extension was
+    ; built for another version of Pillow" at runtime. config.yaml lives at
+    ; $INSTDIR root and is preserved.
+    RMDir /r "$INSTDIR\python"
+    RMDir /r "$INSTDIR\app"
+    RMDir /r "$INSTDIR\scripts"
+    Delete "$INSTDIR\${EXENAME}"
+    Delete "$INSTDIR\VERSION"
+    Delete "$INSTDIR\config.example.yaml"
+
     ; Copy entire distribution
     File /r "dist\media-server\*.*"