fix(regex_validator): add ReDoS detection via regexploit

Detect catastrophic backtracking patterns before regex compilation
using regexploit library. Add ReDoSDetectedError exception and
_MINIMUM_STARRINESS threshold (>=3) to catch dangerous patterns
like (a+)+b. Update pyproject.toml deps, add tests for detection.

backend/pyproject.toml

@@ -19,6 +19,7 @@ dependencies = [
     "bcrypt>=4.2.0",
     "geoip2>=4.8.0",
     "prometheus-client>=0.21.0",
+    "regexploit>=1.0.0",
 ]
 
 [project.optional-dependencies]