[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/api/routes/me.py

@@ -2,6 +2,7 @@ from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.db import get_db
 from app.core.deps import get_current_user
+from app.core.errors import api_error
 from app.core.security import hash_password, verify_password
 from app.models.models import User
 from app.schemas.user import UserOut, UserPasswordChange
@@ -22,10 +23,16 @@ async def change_password(
     db: AsyncSession = Depends(get_db),
 ) -> dict:
     if not verify_password(payload.current_password, user.password_hash):
-        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="Current password is incorrect")
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=api_error("invalid_current_password", "Current password is incorrect"),
+        )
 
     if verify_password(payload.new_password, user.password_hash):
-        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="New password must be different")
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=api_error("password_reuse_not_allowed", "New password must be different"),
+        )
 
     user.password_hash = hash_password(payload.new_password)
     await db.commit()