Fix Concrete ML compatibility issue and improve FHE service logging

- Update requirements.txt to make concrete-ml optional with clear comment
- Enhance FHE service warning message to include Python version info
- Remove concrete-ml from mandatory dependencies due to Python 3.13 incompatibility
- Maintain full functionality with TenSEAL provider as default

This resolves the 'Concrete ML not installed' warning by providing clear
information about the version compatibility issue while preserving all
FHE functionality through the TenSEAL provider.

apps/coordinator-api/requirements.txt

@@ -14,7 +14,7 @@ aiosqlite>=0.20.0
 sqlmodel>=0.0.16
 numpy>=1.26.0
 tenseal
-concrete-ml
+# concrete-ml  # Optional - requires Python <3.13, currently incompatible
 
 # HTTP & Networking
 httpx>=0.27.0

apps/coordinator-api/src/app/services/fhe_service.py

@@ -210,8 +210,10 @@ class FHEService:
         # Optional Concrete ML provider
         try:
             providers["concrete"] = ConcreteMLProvider()
-        except ImportError:
-            logging.warning("Concrete ML not installed; skipping Concrete provider")
+        except ImportError as e:
+            logging.warning("Concrete ML not installed; skipping Concrete provider. "
+                          "Concrete ML requires Python <3.13. Current version: %s", 
+                          __import__('sys').version.split()[0])
 
         self.providers = providers
         self.default_provider = "tenseal"