feat: opt-in state.db sync for /insights visibility (#92)

WebUI sessions were invisible to 'hermes /insights' because the WebUI
bypasses the gateway and calls AIAgent.run_conversation() directly,
never writing to state.db.

New 'Sync usage to /insights' setting (default: off) that mirrors
WebUI session metadata (tokens, cost, model, title) into state.db
after each turn. Uses absolute token counts to avoid double-counting.

Components:
- api/state_sync.py: bridge module with sync_session_start() and
  sync_session_usage(). Uses ensure_session() (idempotent) and
  update_token_counts(absolute=True). All wrapped in try/except.
- api/config.py: new 'sync_to_insights' boolean setting
- api/streaming.py: calls sync_session_usage() after s.save()
- api/routes.py: same for the non-streaming chat path
- Settings UI: checkbox toggle with description

Default off because:
- Writing to state.db while CLI/gateway also writes could cause
  WAL lock contention on busy systems
- Some users may not want WebUI sessions in /insights stats

Closes #92

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/routes.py

@@ -995,6 +995,20 @@ def _handle_chat_sync(handler, body):
         else: os.environ['HERMES_SESSION_KEY'] = old_session_key
     s.messages = result.get('messages') or s.messages
     s.title = title_from(s.messages, s.title); s.save()
+    # Sync to state.db for /insights (opt-in setting)
+    try:
+        if load_settings().get('sync_to_insights'):
+            from api.state_sync import sync_session_usage
+            sync_session_usage(
+                session_id=s.session_id,
+                input_tokens=s.input_tokens or 0,
+                output_tokens=s.output_tokens or 0,
+                estimated_cost=s.estimated_cost,
+                model=s.model,
+                title=s.title,
+            )
+    except Exception:
+        pass
     return j(handler, {
         'answer': result.get('final_response') or '',
         'status': 'done' if result.get('completed', True) else 'partial',