Reduce Web UI render overhead by reusing RbelHtmlRenderingToolkit

[svbergmann]

• Reused a single RbelHtmlRenderingToolkit per /getMessagesWithHtml request instead of constructing a new one per message, and added a reset hook to clear per‑message JSON note tags. This reduces object churn and prevents note tag accumulation across messages.
• Ensured full HTML document rendering resets the note tags between messages too, so output stays correct and memory stays bounded.

Why this works for performance:

• The toolkit holds an ObjectMapper and a noteTags map that grows during rendering. Creating a new toolkit for every message or letting noteTags grow across messages increases allocations and GC pressure.
• Reusing one toolkit per request avoids repeated heavyweight instantiation.
• Resetting noteTags before each message keeps memory bounded and prevents mismatched note replacements across messages, preserving correct rendering.