refactor: unify ModelType into common/model_types.rs

Consolidate 4 separate ModelType enum definitions (ml 15 variants,
model_loader 7, campaign 2, job_spawner 4) into a single canonical
definition in common/src/model_types.rs with the union of all variants
and all methods (file_extension, as_str, to_db_string, weight, from_str,
Display).

- ml/src/lib.rs: replace 15-variant enum with re-export
- model_loader/src/lib.rs: replace 7-variant enum with re-export,
  update PascalCase names (Dqn->DQN, Tft->TFT, etc)
- ml/hyperopt/campaign.rs: replace 2-variant enum with re-export
- services/ml_training_service/job_spawner.rs: replace 4-variant enum
  with re-export, MAMBA2->MAMBA
- Remove orphan impl ToString in ml/observability/metrics.rs (Display
  now provided by canonical type)
- Update backtesting_service and model_loader tests for new names

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

model_loader/src/lib.rs

@@ -17,8 +17,8 @@ use std::time::SystemTime;
 use storage::{ObjectStoreBackend, Storage};
 use tracing::{debug, info, warn};
 
-// Re-export canonical ModelType from ml crate (single source of truth)
-pub use ml::ModelType;
+// Re-export canonical ModelType from common crate
+pub use common::model_types::ModelType;
 
 /// Model metadata for version tracking
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -373,10 +373,17 @@ mod tests {
     use super::*;
 
     #[test]
+<<<<<<< HEAD
     fn test_model_type_s3_prefix() {
         assert_eq!(ModelType::TLOB.s3_prefix(), "tlob_transformer");
         assert_eq!(ModelType::DQN.s3_prefix(), "dqn");
         assert_eq!(ModelType::MAMBA.s3_prefix(), "mamba2");
+=======
+    fn test_model_type_as_str() {
+        assert_eq!(ModelType::TLOB.as_str(), "tlob_transformer");
+        assert_eq!(ModelType::DQN.as_str(), "dqn");
+        assert_eq!(ModelType::MAMBA.as_str(), "mamba2");
+>>>>>>> dd4532a3 (refactor: unify ModelType into common/model_types.rs)
     }
 
     #[test]