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/llm.py

@@ -159,6 +159,24 @@ async def wait_for_model_loaded(model: str, timeout: float = 90.0) -> bool:
         await asyncio.sleep(min(2.0, remaining))
 
 
+async def _raise_ollama_error(resp: httpx.Response, model: str) -> None:
+    """On a non-2xx Ollama response, log its body before raising.
+
+    ``resp.raise_for_status()`` alone throws away Ollama's error body, which
+    carries the actual reason (e.g. ``"model does not support tools"``,
+    ``"context length exceeded"``). Reading the body first means failures
+    surface a usable message instead of a bare HTTPStatusError.
+    """
+    if resp.status_code < 400:
+        return
+    body = (await resp.aread()).decode("utf-8", errors="replace").strip()
+    logger.error(
+        "Ollama /api/chat %d for model=%s: %s",
+        resp.status_code, model, body[:500],
+    )
+    resp.raise_for_status()
+
+
 async def stream_chat(
     messages: list[dict],
     model: str,
@@ -184,7 +202,7 @@ async def stream_chat(
             f"{Config.OLLAMA_URL}/api/chat",
             json=payload,
         ) as resp:
-            resp.raise_for_status()
+            await _raise_ollama_error(resp, model)
             async for line in resp.aiter_lines():
                 if not line.strip():
                     continue
@@ -245,7 +263,7 @@ async def stream_chat_with_tools(
             f"{Config.OLLAMA_URL}/api/chat",
             json=payload,
         ) as resp:
-            resp.raise_for_status()
+            await _raise_ollama_error(resp, model)
             accumulated_tool_calls: list[dict] = []
             async for line in resp.aiter_lines():
                 if not line.strip():