feat: in-app live log (SSE activity feed)

Turn the inert "Show Work" thinking panel into a real live activity log:
- lyra/logbus.py: thread-safe in-memory ring buffer other modules publish to
- chat.respond logs backend/model/embed per turn, recall counts, reply size;
  web layer logs chat errors
- server: replace the keep-alive /stream/thinking stub with /stream/logs, an
  SSE endpoint that replays the recent buffer then streams new events
- UI: repurpose the panel as a global "Live Log" — connects on load, renders
  level/time/msg/fields, drops the old per-session localStorage + dead popup

Every turn now shows its backend + model in-app, so local-vs-cloud (free vs
paid) is visible at a glance.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/logbus.py

@@ -0,0 +1,31 @@
+"""In-memory live log bus.
+
+A thread-safe ring buffer that any part of Lyra can publish to and the web
+server streams to the browser over SSE. Deliberately process-local and
+ephemeral — it's an activity feed, not durable logging.
+"""
+from __future__ import annotations
+
+import threading
+import time
+from collections import deque
+
+_LOCK = threading.Lock()
+_EVENTS: deque[dict] = deque(maxlen=500)
+_SEQ = 0
+
+
+def log(level: str, msg: str, **fields) -> None:
+    """Publish an event. `level` is info/debug/error/system; fields are extras."""
+    global _SEQ
+    with _LOCK:
+        _SEQ += 1
+        _EVENTS.append(
+            {"seq": _SEQ, "ts": time.time(), "level": level, "msg": msg, "fields": fields}
+        )
+
+
+def since(seq: int) -> list[dict]:
+    """All buffered events with seq greater than `seq` (for SSE catch-up/polling)."""
+    with _LOCK:
+        return [e for e in _EVENTS if e["seq"] > seq]