feat(ai-services): complete AVM v2 ensemble — upload endpoint, per-district metrics, A/B routing

- Add POST /avm/v2/upload-training-data so AvmRetrainCronService can push
  CSV rows before triggering retraining (was called but missing)
- Add per-district MAE/MAPE/RMSE/R² to _evaluate_ensemble output;
  district_metrics are now returned in AVMv2TrainResponse and stored
  separately from global metrics in the model registry
- Add predict_with_ab() that applies the active model's ab_test_traffic_pct
  for deterministic per-property cohort assignment (v2 vs heuristic baseline)
- Add POST /avm/v2/ab-config to set traffic_pct on the active registry entry
- Add AVMv2ABConfigRequest schema
- Expand test suite: 24 → 28 tests covering upload, A/B config, and new
  validation paths; all green

Co-Authored-By: Paperclip <noreply@paperclip.ing>

libs/ai-services/app/models/avm_v2.py

@@ -213,6 +213,15 @@ class AVMv2RollbackRequest(BaseModel):
     target_version: str = Field(..., min_length=1, description="Model version to roll back to")
 
 
+class AVMv2ABConfigRequest(BaseModel):
+    """Request to update the A/B test traffic percentage for the active model."""
+
+    traffic_pct: float = Field(
+        ..., ge=0, le=1,
+        description="Fraction of /predict calls routed to v2 (0=disabled, 0.10=10%, 1=100%)",
+    )
+
+
 class AVMv2FeatureImportanceResponse(BaseModel):
     """Global feature importance across the loaded ensemble.