debug: add exc_info=True + unique logger names to expose real traceback

Both 'Inference error' log lines use identical logger strings, making it
impossible to tell which except block is catching the crash. Also, broad
except blocks were swallowing the full traceback so the real crash site
and cause were hidden.

Changes:
- policy_server.py: logger renamed to 'ml.serving.policy_server';
  predict() and predict_batch() error handlers add exc_info=True
- routes/policy.py: logger renamed to 'routes.policy.decide';
  _infer_from_vector() error handler adds exc_info=True;
  decide() fallback warning adds exc_info=True

The next crash will print the full stack trace, pinpointing the exact
line and call chain causing the TypeError.

ml/serving/policy_server.py

@@ -52,7 +52,8 @@ from ml.config.scope import (
 )
 from ml.encoder.state_encoder import CardEmbeddingIndex, StateEncoder
 
-logger = logging.getLogger("ml.serving")
+# Unique logger name so log lines are attributable to this module specifically
+logger = logging.getLogger("ml.serving.policy_server")
 
 
 def _unwrap_logits(raw):
@@ -175,7 +176,7 @@ class PolicyInferenceService:
 
         except Exception as e:
             self._load_error = str(e)
-            logger.error("Failed to load checkpoint: %s", e)
+            logger.error("Failed to load checkpoint: %s", e, exc_info=True)
             return False
 
     def reload(self, checkpoint_path: str = None) -> bool:
@@ -207,7 +208,7 @@ class PolicyInferenceService:
 
         except Exception as e:
             self._load_error = str(e)
-            logger.error("Hot-reload failed: %s", e)
+            logger.error("Hot-reload failed: %s", e, exc_info=True)
             return False
 
     def predict(
@@ -286,7 +287,7 @@ class PolicyInferenceService:
 
         except Exception as e:
             elapsed_ms = (time.time() - t_start) * 1000
-            logger.error("Inference error: %s", e)
+            logger.error("Inference error: %s", e, exc_info=True)
             return {
                 "error": str(e),
                 "inference_ms": round(elapsed_ms, 2),
@@ -346,7 +347,7 @@ class PolicyInferenceService:
             )
 
         except Exception as e:
-            logger.error("Batch inference error: %s", e)
+            logger.error("Batch inference error: %s", e, exc_info=True)
             results = [{"error": str(e)}] * len(snapshots)
 
         return results