fix(api): 修復全部 lint 錯誤 (ruff --fix)

- Import sorting (I001)
- Unused imports (F401)
- f-string without placeholders (F541)
- Loop variable unused (B007)
- zip() strict parameter (B905)
- Exception chaining (B904)
- collections.abc imports (UP035)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/services/__init__.py

@@ -44,6 +44,14 @@ from .graph_rag import (
     create_mock_topology,
     topology_graph,
 )
+from .model_registry import (
+    IModelRegistry,
+    ModelRegistry,
+    get_model,
+    get_model_by_complexity,
+    get_model_registry,
+    reset_model_registry,
+)
 from .trust_engine import (
     RiskAdjustment,
     RiskLevel,
@@ -99,4 +107,11 @@ __all__ = [
     "ConsensusResult",
     "AgentOpinion",
     "AgentType",
+    # Model Registry (Phase 12 P1)
+    "ModelRegistry",
+    "IModelRegistry",
+    "get_model_registry",
+    "get_model",
+    "get_model_by_complexity",
+    "reset_model_registry",
 ]