PR 2: Fix Duplicate Artifact File Sending Across Turns

Problem

When using the present_files tool to generate file artifacts, the system
sends all historically generated files in the conversation on every new
turn, rather than only newly-produced artifacts.

Additionally, if DeerFlow or its Gateway restarts (new thread_id assigned
to the same IM conversation), all previously sent files are re-sent again
because the sent-artifact tracking was lost.

Root Cause

The original _extract_artifacts function was designed to scan the message
history and extract file paths from present_files tool calls after the
last human message only. However:

1. The original implementation had a subtle bug: it iterated messages in
   reverse and stopped at the first type == "human" message, but
   present_files tool calls in subsequent turns could be missed if
   the reverse scan logic was incorrect.
2. There was no persistence of already-sent artifacts — every turn
   re-extracted artifacts from the message history, which contained all
   past tool calls.
3. After a restart, the ChannelStore's thread_id mapping changed but
   there was no cross-restart tracking of sent artifacts.

Fix

store.py — New sent-artifact persistence API

• Added get_sent_artifacts(channel_name, chat_id, topic_id=None) and
  set_sent_artifacts(channel_name, chat_id, paths, topic_id=None) methods.
• Keys are stored as sent_artifacts:<channel>:<chat_id>[:<topic_id>] in
  the existing JSON-backed ChannelStore, ensuring survival across server
  restarts.
• Keyed by IM conversation identity (channel + chat), not by internal
  thread_id, so the record persists even when Gateway restarts assign a
  new internal thread.

manager.py — New artifact diff logic

• Replaced the reverse-scan approach with a state-based extraction from
  the artifacts state key (already normalized by present_file_tool).
• Added _diff_artifacts() function that compares extracted artifact paths
  against the store's sent_artifacts record for that conversation, returning
  only newly-produced artifacts.
• Updated both _handle_chat and _handle_streaming_chat to pass
  channel_name, chat_id, topic_id, and store to the new diff logic.

Usage Flow

1. On first turn with artifacts: all artifacts are sent, recorded in store.
2. On subsequent turns: only artifacts NOT in the store are sent.
3. After restart: new thread_id, but sent_artifacts lookup uses
   channel:chat_id → previous records found → no duplicates sent.

Testing

• Production-verified with 4 historical artifacts, only 2 new artifacts
  sent on subsequent turns.
• Verified cross-restart persistence: after service restart, sent-artifact
  records were correctly loaded from store.json.

Files Changed

• backend/app/channels/store.py
• backend/app/channels/manager.py