[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/admin_users.py

@@ -3,6 +3,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from app.core.db import get_db
 from app.core.deps import require_roles
+from app.core.errors import api_error
 from app.core.security import hash_password
 from app.models.models import User
 from app.schemas.user import UserCreate, UserOut, UserUpdate
@@ -22,7 +23,7 @@ async def list_users(admin: User = Depends(require_roles("admin")), db: AsyncSes
 async def create_user(payload: UserCreate, admin: User = Depends(require_roles("admin")), db: AsyncSession = Depends(get_db)) -> UserOut:
     exists = await db.scalar(select(User).where(User.email == payload.email))
     if exists:
-        raise HTTPException(status_code=409, detail="Email already exists")
+        raise HTTPException(status_code=409, detail=api_error("email_exists", "Email already exists"))
     user = User(
         email=payload.email,
         first_name=payload.first_name,
@@ -46,13 +47,13 @@ async def update_user(
 ) -> UserOut:
     user = await db.scalar(select(User).where(User.id == user_id))
     if not user:
-        raise HTTPException(status_code=404, detail="User not found")
+        raise HTTPException(status_code=404, detail=api_error("user_not_found", "User not found"))
     update_data = payload.model_dump(exclude_unset=True)
     next_email = update_data.get("email")
     if next_email and next_email != user.email:
         existing = await db.scalar(select(User).where(User.email == next_email))
         if existing and existing.id != user.id:
-            raise HTTPException(status_code=409, detail="Email already exists")
+            raise HTTPException(status_code=409, detail=api_error("email_exists", "Email already exists"))
     if "password" in update_data:
         raw_password = update_data.pop("password")
         if raw_password:
@@ -68,10 +69,10 @@ async def update_user(
 @router.delete("/{user_id}")
 async def delete_user(user_id: int, admin: User = Depends(require_roles("admin")), db: AsyncSession = Depends(get_db)) -> dict:
     if user_id == admin.id:
-        raise HTTPException(status_code=400, detail="Cannot delete yourself")
+        raise HTTPException(status_code=400, detail=api_error("cannot_delete_self", "Cannot delete yourself"))
     user = await db.scalar(select(User).where(User.id == user_id))
     if not user:
-        raise HTTPException(status_code=404, detail="User not found")
+        raise HTTPException(status_code=404, detail=api_error("user_not_found", "User not found"))
     await db.delete(user)
     await db.commit()
     await write_audit_log(db, "admin.user.delete", admin.id, {"deleted_user_id": user_id})