Fix: bind memory message user_id to authenticated user for JWT auth (#14745)

### Related issues

Closes #14744

### What problem does this PR solve?

The Memory REST endpoint `POST /api/v1/messages` previously persisted
whatever `user_id` the client sent in the JSON body. Memory rows were
therefore attributed to an arbitrary string, even when the caller
authenticated as a normal workspace user via JWT (browser/session-style
bearer token decoded into an access token). That broke attribution and
audit semantics for shared memories (team visibility): any authorized
writer could spoof another subject id.

The Python SDK already sends an optional `user_id` for integrations
using **API keys** (`APIToken`) to tag an external subject distinct from
the tenant owner user.

### Solution

- Record **`g.auth_via_api_token`** in `_load_user`
(`api/apps/__init__.py`): set `True` only when authentication resolves
via `APIToken`, otherwise `False` after JWT-based login succeeds.
- In **`POST /messages`** (`memory_api.add_message`): if the request was
authenticated with an API key, keep accepting optional `user_id` from
the body (default empty string). For JWT-authenticated users, **always**
set stored `user_id` to **`current_user.id`** and ignore the client
field.
- Guard reads of `g` with **`RuntimeError`** handling so isolated
imports or tests without a Quart application context do not fail when
resolving `user_id`.
- Document on **`RAGFlow.add_message`** that `user_id` is only
meaningful for API-key authentication.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

### Testing

- `python -m py_compile` on modified modules (`api/apps/__init__.py`,
`api/apps/restful_apis/memory_api.py`).
- Recommended: run web/SDK memory message tests (`test_add_message`,
`test_message_routes_unit`) against a full environment with `quart` and
configured services.

### Notes for reviewers

- Behavior change **only** for callers using JWT-style authorization on
`POST /messages`; API-key callers keep prior optional `user_id`
semantics.

Co-authored-by: jony376 <jony376@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

sdk/python/ragflow_sdk/ragflow.py

@@ -334,6 +334,7 @@ class RAGFlow:
             raise Exception(res["message"])
 
     def add_message(self, memory_id: list[str], agent_id: str, session_id: str, user_input: str, agent_response: str, user_id: str = "") -> str:
+        """Append messages to memories; ``user_id`` is forwarded only for API-key auth (external subject)."""
         payload = {
             "memory_id": memory_id,
             "agent_id": agent_id,