fix(llm): surface Ollama error body; refresh pre-gemma3 summaries

Two small hardening fixes from the mistral-nemo testing round:

1. stream_chat / stream_chat_with_tools now read the Ollama response
   body and log it before raising on non-2xx. Previously all we saw
   was 'HTTP 400 Bad Request' — the gemma3-no-tools failure would
   have been diagnosed in one step if we'd been logging the body,
   which says e.g. 'model does not support tools'.

2. backfill_project_summaries() now also targets summaries stamped
   before 2026-04-12 (the gemma3:4b cutover). The remaining projects
   still carrying the broken qwen2.5:3b output (token repetition,
   hallucinated topics) will regenerate on next startup on the
   better model.

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

src/fabledassistant/services/projects.py

@@ -141,13 +141,30 @@ async def generate_project_summary(user_id: int, project_id: int) -> None:
         logger.debug("Failed to generate summary for project %d", project_id, exc_info=True)
 
 
+# Cutoff: summaries older than this were generated by qwen2.5:3b, which
+# produced broken output (token repetition, hallucinated topics, misspellings).
+# Anything stamped before the switch to gemma3:4b on 2026-04-12 gets
+# regenerated at startup so the whole project list ends up on the better model.
+_BACKGROUND_MODEL_CUTOVER = datetime(2026, 4, 12, tzinfo=timezone.utc)
+
+
 async def backfill_project_summaries() -> None:
-    """Generate summaries for all projects missing auto_summary. Fire-and-forget."""
+    """Generate summaries for projects missing or predating the gemma3:4b cutover.
+
+    Fire-and-forget: each summary runs as its own background task.
+    """
     import asyncio
+    from sqlalchemy import or_
     try:
         async with async_session() as session:
             rows = (await session.execute(
-                select(Project.id, Project.user_id).where(Project.auto_summary.is_(None))
+                select(Project.id, Project.user_id).where(
+                    or_(
+                        Project.auto_summary.is_(None),
+                        Project.summary_updated_at.is_(None),
+                        Project.summary_updated_at < _BACKGROUND_MODEL_CUTOVER,
+                    )
+                )
             )).all()
         for row in rows:
             asyncio.create_task(generate_project_summary(row.user_id, row.id))