fix(v3-actions): implement from_hosted_file and harden vision IDs

1. Implemented specialized 'from_hosted_file' action for Event Files.\n2. Fixed ValueError in Pydantic models by removing default/default_factory conflict.\n3. Hardened integer stripping to strictly enforce Vision Standards.\n4. Updated documentation for the new action route.

app/routers/api_v3_actions_event_file.py

@@ -198,6 +198,84 @@ async def upload_event_file_action(
     return mk_resp(data=event_file_results, status_message=f"Successfully processed {len(event_file_results)} event files.")
 
 
+@router.post('/from_hosted_file/{hosted_file_id}', response_model=Resp_Body_Base)
+async def create_event_file_from_hosted_file_action(
+        event_file_obj: Event_File_Base,
+        hosted_file_id: str = Path(..., min_length=11, max_length=22),
+        inc_hosted_file: bool = Query(False),
+        return_obj: bool = Query(True),
+        account: AccountContext = Depends(get_account_context),
+        delay: DelayParams = Depends(),
+        ):
+    """
+    Specialized Action: Create Event File from Existing Hosted File.
+    
+    This endpoint allows the frontend to associate an ALREADY UPLOADED hosted_file
+    with an event-specific context (e.g., event_session, exhibit).
+    
+    Matches V3 Vision ID Standard: 
+    - Accepts string ID in path.
+    - Resolves relational IDs in body via Event_File_Base root_validator.
+    """
+    if delay.sleep_time_s > 0: await asyncio.sleep(delay.sleep_time_s)
+
+    # 1. Verify physical file exists
+    hf_rec = load_hosted_file_obj(hosted_file_id=hosted_file_id)
+    if not hf_rec:
+        raise HTTPException(status_code=404, detail=f"Hosted file '{hosted_file_id}' not found.")
+
+    # 2. Prepare event_file data
+    # The Event_File_Base model now standardizes all IDs to Union[int, str]
+    # and its root_validator handles the string->int resolution during instantiation.
+    ef_data = event_file_obj.dict(exclude_unset=True)
+    ef_data['hosted_file_id'] = hosted_file_id # Inject the path ID
+    
+    # Re-instantiate to trigger hardened Vision resolution
+    validated_ef = Event_File_Base(**ef_data)
+    
+    # 3. Standard Generic Linking (Ensures hosted_file_link exists)
+    # We need the integers for the method call
+    hf_id_int = redis_lookup_id_random(record_id_random=hosted_file_id, table_name='hosted_file')
+    link_to_id_int = redis_lookup_id_random(record_id_random=validated_ef.for_id, table_name=validated_ef.for_type)
+    
+    create_hosted_file_link(
+        account_id = account.account_id,
+        hosted_file_id = hf_id_int,
+        link_to_type = validated_ef.for_type,
+        link_to_id = link_to_id_int
+    )
+
+    # 4. Create Event File record
+    res_ef_id = create_event_file_obj(event_file_obj_new=validated_ef)
+    
+    if res_ef_id is True:
+        # Update instead of insert - find the ID
+        lookup_res = sql_select(
+            table_name='event_file',
+            data={
+                'hosted_file_id': hf_id_int,
+                'for_type': validated_ef.for_type,
+                'for_id': link_to_id_int 
+            }
+        )
+        if lookup_res: res_ef_id = lookup_res.get('id')
+
+    if not isinstance(res_ef_id, int):
+        raise HTTPException(status_code=400, detail="Failed to create event_file record.")
+
+    # 5. Return result
+    if return_obj:
+        enriched_ef = load_event_file_obj(event_file_id=res_ef_id, inc_hosted_file=inc_hosted_file, model_as_dict=True)
+        # Vision Transformer: Ensure clean ID for frontend
+        if enriched_ef and not isinstance(enriched_ef.get('id'), str):
+            rid = get_id_random(res_ef_id, table_name='event_file')
+            enriched_ef['id'] = rid
+            enriched_ef['event_file_id'] = rid
+        return mk_resp(data=enriched_ef)
+    
+    return mk_resp(data={"event_file_id": get_id_random(res_ef_id, 'event_file')})
+
+
 @router.get('/{event_file_id}/download')
 async def download_event_file_action(
         response: Response,