fix(api): address review on the event-loop streams

- Per-user SSE cap uses per-connection leases in a sorted set instead of
  a shared INCR/DECR counter with a TTL. A stream that outlived the TTL
  could let the counter expire, then decrement another stream's slot or
  drive the count negative. Leases refresh while a stream sends frames
  and age out when a stream dies without cleanup.
- Shielded cleanup is bounded per step: stream close and on_close in
  ClosingStreamingResponse, unsubscribe and close in AsyncTopic, lease
  release, and the replay-budget check, so a dead Redis connection can't
  hold a request or a graceful shutdown.
- Artifact downloads send an ASCII filename plus an RFC 5987 filename*
  for non-ASCII names, build headers before opening the file, and close
  disk handles off the event loop.
- Type hints and docstrings on the new helpers.

tests/test_events_substrate.py

@@ -19,7 +19,7 @@ from docsgpt.api.events.routes import (
     _normalize_last_event_id,
 )
 from docsgpt.events.keys import (
-    connection_counter_key,
+    connection_leases_key,
     stream_id_compare,
     stream_key,
     topic_name,
@@ -39,8 +39,8 @@ class TestKeys:
     def test_topic_name(self):
         assert topic_name("alice") == "user:alice"
 
-    def test_connection_counter_key(self):
-        assert connection_counter_key("alice") == "user:alice:sse_count"
+    def test_connection_leases_key(self):
+        assert connection_leases_key("alice") == "user:alice:sse_leases"
 
 
 # ── stream_id_compare ───────────────────────────────────────────────────