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_buffer.py

@@ -70,7 +70,7 @@ class GenerationBuffer:
 
 
 # Module-level singleton registry
-_buffers: dict[int, GenerationBuffer] = {}
+_buffers: dict[int | str, GenerationBuffer] = {}
 _cleanup_task: asyncio.Task | None = None
 _GRACE_PERIOD = 60.0  # seconds to keep completed buffers
 
@@ -92,20 +92,38 @@ def remove_buffer(conv_id: int) -> None:
     _buffers.pop(conv_id, None)
 
 
+def create_assist_buffer(user_id: int) -> GenerationBuffer:
+    key = f"assist:{user_id}"
+    existing = _buffers.get(key)
+    if existing and existing.state == GenerationState.RUNNING:
+        raise RuntimeError(f"Assist generation already running for user {user_id}")
+    buf = GenerationBuffer(conversation_id=0, assistant_message_id=0)
+    _buffers[key] = buf
+    return buf
+
+
+def get_assist_buffer(user_id: int) -> GenerationBuffer | None:
+    return _buffers.get(f"assist:{user_id}")
+
+
+def remove_assist_buffer(user_id: int) -> None:
+    _buffers.pop(f"assist:{user_id}", None)
+
+
 async def _cleanup_loop() -> None:
     """Remove completed/errored buffers after grace period."""
     while True:
         await asyncio.sleep(15)
         now = time.monotonic()
         to_remove = [
-            cid for cid, buf in _buffers.items()
+            key for key, buf in _buffers.items()
             if buf.state != GenerationState.RUNNING
             and buf.finished_at is not None
             and now - buf.finished_at > _GRACE_PERIOD
         ]
-        for cid in to_remove:
-            _buffers.pop(cid, None)
-            logger.debug("Cleaned up generation buffer for conversation %d", cid)
+        for key in to_remove:
+            _buffers.pop(key, None)
+            logger.debug("Cleaned up generation buffer for key %s", key)
 
 
 def start_cleanup_loop() -> None: