fix(discuss): hard-fail empty articles and skip RAG on seed turn

Discuss flow was hallucinating unrelated content when article
extraction returned empty or RAG pulled in orphan notes that looked
more relevant than the generic seed prompt.

- seed_article_discussion raises EmptyArticleError on empty body;
  briefing and /news routes return 422 instead of staging an empty
  synthetic tool result.
- build_context skips RAG auto-injection when user_message matches
  ARTICLE_DISCUSS_SEED so the article IS the context on turn one;
  follow-up turns keep RAG on.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/routes/chat.py

@@ -521,7 +521,10 @@ async def create_conversation_from_article(item_id: int):
     from sqlalchemy import select as _select
     from fabledassistant.models import async_session as _async_session
     from fabledassistant.models.rss_feed import RssItem, RssFeed
-    from fabledassistant.services.article_context import seed_article_discussion
+    from fabledassistant.services.article_context import (
+        EmptyArticleError,
+        seed_article_discussion,
+    )
 
     uid = get_current_user_id()
 
@@ -540,7 +543,16 @@ async def create_conversation_from_article(item_id: int):
     conv = await create_conversation(uid, title=conv_title, conversation_type="chat")
 
     model = await get_setting(uid, "default_model", "") or Config.OLLAMA_MODEL
-    discuss_prompt = await seed_article_discussion(conv.id, item, model)
+    try:
+        discuss_prompt = await seed_article_discussion(conv.id, item, model)
+    except EmptyArticleError as e:
+        # Roll back the empty conversation so the user doesn't end up with a
+        # phantom entry in their chat list.
+        try:
+            await delete_conversation(uid, conv.id)
+        except Exception:
+            logger.warning("Failed to clean up empty article conversation %s", conv.id)
+        return jsonify({"error": str(e)}), 422
 
     # Reload conversation so we see the two messages the helper just added.
     conv = await get_conversation(uid, conv.id)