Rename the trigger node type to debounce, migrating stored flows

The store rewrites `"type": "trigger"` to `debounce` in every published and
draft flow when it opens, bumps the version and commits once. No alias stays
behind, so `trigger` is now an unknown type.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

frontend/src/components/Flow/FlowNode.tsx

@@ -55,7 +55,7 @@ const NODE_ICONS = {
   change: Shuffle,
   rbe: Filter,
   join: Merge,
-  trigger: Timer,
+  debounce: Timer,
   exec: Terminal,
   file: FileText,
   ntfy: Bell,