feat: refactor to FastAPI architecture v2.0

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

app/models/responses.py

@@ -0,0 +1,22 @@
+"""API response models."""
+
+from typing import Any, List, Optional
+
+from pydantic import BaseModel
+
+
+class HealthResponse(BaseModel):
+    status: str = "ok"
+    version: str
+
+
+class ValidationResponse(BaseModel):
+    valid: bool
+    errors: List[str] = []
+
+
+class ImportExcelResponse(BaseModel):
+    success: bool
+    document: Optional[Any] = None
+    errors: List[str] = []
+    warnings: List[str] = []