feat(#11): AdminEvents pane — GET /admin/events SSE (session-filtered)

v1 coverage-audit: the last unbuilt design-brief §5 debug pane. #11's
blocker was already satisfied (admin key carries admin.events.read).
Completes the admin/debug-observability core.

- sse_client.py: AdminEvent dataclass + stream_admin_events — a new
  long-lived SSE consumer for the admin lifecycle stream (envelope
  {id,type,timestamp,data}), admin-scoped (bearer-override), Last-Event-ID
  resume. non-200 -> SseConnectFailed; mid-drop -> SseConnectionDropped.
- tui.py: "AdminEvents" TabPane + _format_admin_event + _admin_event_matches
  (design-brief §6 filter: active-session + non-heartbeat system.*) +
  _stream_admin_events long-lived best-effort worker (unconditional
  on_mount; self-labels not-configured / unavailable / stream-ended).
- Contract-skipped for stream_admin_events (out of #1's turn-SSE scope;
  spec § Admin Event Stream is the reference). TDD: 4 sse_client + 5 tui
  tests. Suite 561 green.
- LIVE-AUTH-PROVEN on :8081 (GET /admin/events -> HTTP 200 under admin key).

Coverage: REST 12/40. Tier 1 debug-observability core complete.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.18.10"
+version = "0.18.11"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"