Fix Python 3.10 compatibility (StrEnum requires 3.11)

enum.StrEnum was added in Python 3.11, but the documented minimum is
  3.10. Add a compatibility shim in backend/app/core/compat.py that falls
  back to (str, Enum) on older versions. Updated all 5 import sites and
  lowered pyproject.toml target-version to py310.

install/install.sh

@@ -242,7 +242,7 @@ detect_python() {
     minor=$(echo "$version" | cut -d'.' -f2)
 
     if [[ "$major" -lt 3 ]] || { [[ "$major" -eq 3 ]] && [[ "$minor" -lt 10 ]]; }; then
-        log_warn "Python $version found, but 3.10+ is required"
+        log_warn "Python $version found, but 3.10 or newer is required"
         return 1
     fi