[NX-101 Issue] Refactor error handling to use consistent API error format

Replaced all inline error messages with the standardized `api_error` helper for consistent error response formatting. This improves clarity, maintainability, and ensures uniform error structures across the application. Updated logging for collector failures to include error class and switched to warning level for target unreachable scenarios.

backend/app/core/deps.py

@@ -5,6 +5,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.config import get_settings
 from app.core.db import get_db
+from app.core.errors import api_error
 from app.models.models import User
 
 settings = get_settings()
@@ -16,27 +17,42 @@ async def get_current_user(
     db: AsyncSession = Depends(get_db),
 ) -> User:
     if not credentials:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Missing token")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("missing_token", "Missing token"),
+        )
     token = credentials.credentials
     try:
         payload = jwt.decode(token, settings.jwt_secret_key, algorithms=[settings.jwt_algorithm])
     except jwt.InvalidTokenError as exc:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token") from exc
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_token", "Invalid token"),
+        ) from exc
 
     if payload.get("type") != "access":
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token type")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("invalid_token_type", "Invalid token type"),
+        )
 
     user_id = payload.get("sub")
     user = await db.scalar(select(User).where(User.id == int(user_id)))
     if not user:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not found")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail=api_error("user_not_found", "User not found"),
+        )
     return user
 
 
 def require_roles(*roles: str):
     async def role_dependency(user: User = Depends(get_current_user)) -> User:
         if user.role not in roles:
-            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Forbidden")
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail=api_error("forbidden", "Forbidden"),
+            )
         return user
 
     return role_dependency