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/streaming.py

@@ -309,6 +309,21 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                             m['attachments'] = attachments
                             break
             s.save()
+            # Sync to state.db for /insights (opt-in setting)
+            try:
+                from api.config import load_settings as _load_settings
+                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=model,
+                        title=s.title,
+                    )
+            except Exception:
+                pass  # never crash the stream for sync failures
             usage = {'input_tokens': input_tokens, 'output_tokens': output_tokens, 'estimated_cost': estimated_cost}
             # Include context window data from the agent's compressor for the UI indicator
             _cc = getattr(agent, 'context_compressor', None)