Refactor AI Assist to background-task + buffer architecture

The assist flow previously tied the entire LLM generation to a single
POST request with no keepalives, causing NS_ERROR_NET_PARTIAL_TRANSFER
in Firefox when Hypercorn closed the connection during gaps between
chunks. This refactor decouples generation into a background task with
a buffer and a separate SSE stream — the same pattern used by chat.

- generation_buffer.py: Widen _buffers to support string keys, add
  create/get/remove_assist_buffer() using "assist:{user_id}" keys,
  fix cleanup log format for string keys
- generation_task.py: Add run_assist_generation() — lightweight
  background task with no DB persistence or title generation
- notes.py: Replace single POST SSE route with POST /api/notes/assist
  (returns 202) + GET /api/notes/assist/stream (SSE with 15s keepalives
  and Last-Event-ID reconnection); 409 if already running
- useAssist.ts: Switch from apiStreamPost to apiPost + apiSSEStream
  two-step pattern with named event mapping and stream handle cleanup

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

src/fabledassistant/services/generation_task.py

@@ -134,3 +134,25 @@ async def run_generation(
         buf.state = GenerationState.ERRORED
         buf.finished_at = time.monotonic()
         buf.append_event("error", {"error": str(e)})
+
+
+async def run_assist_generation(
+    buf: GenerationBuffer,
+    messages: list[dict],
+    model: str,
+) -> None:
+    """Stream LLM response for assist into buffer. No DB persistence."""
+    try:
+        async for chunk in stream_chat(messages, model, options={"num_predict": 4096}):
+            buf.content_so_far += chunk
+            buf.append_event("chunk", {"chunk": chunk})
+
+        buf.state = GenerationState.COMPLETED
+        buf.finished_at = time.monotonic()
+        buf.append_event("done", {"done": True, "full_text": buf.content_so_far})
+
+    except Exception as e:
+        logger.exception("Error in assist generation task")
+        buf.state = GenerationState.ERRORED
+        buf.finished_at = time.monotonic()
+        buf.append_event("error", {"error": str(e)})