Final cleanup: lazy logfmt build, docstring and example fixes

- transaction.py only builds the logfmt chain when logging is enabled
- callbacks.py module docstring documents the logemit special case
- docs logemit example uses the settable header instead of a user lookup
  that would break when logfmt resolution is registered

docs/database.md

@@ -269,9 +269,9 @@ def resolve_user_key(value: str) -> str | None:
 def emit(ev: LogEvent):
     if ev.kind != "change":
         return default_emit(ev)  # delegate, no chaining needed
-    actor = ev.current.get("users", {}).get(ev.user, {}).get("name", ev.user)
-    line = Line().user(actor, width=20)(" ").action(ev.action)
-    ev.logger.log(ev.level, f"{line}\n" + "\n".join(ev.diff_lines))
+    # Restyle the header; default_emit keeps routing the diff body.
+    ev.header = str(Line().user(ev.user or "-", width=20)(" ").action(ev.action))
+    return True
 ```
 
 #### Terminal Formatting Helpers