Include 'by user' in abort messages

The aborted event now carries the transaction user, resolved through the
logfmt chain against the pre-transaction state so it matches change
headers. Default rendering: '<action> by <user> transaction aborted: <e>'.
User resolution factored into _build_logfmt/_resolve_user helpers shared
by the change and abort paths.

docs/database.md

@@ -234,8 +234,10 @@ def resolve_user_key(value: str) -> str | None:
   `"migrated"`, `"aborted"`), the preferred `logger` and `level`, and all
   relevant state: `action`, `user`, `extra`, `error` (for aborted
   transactions), `diff`, `previous`/`current` state dicts, the built `logfmt`
-  chain, and version info for migration events.  Pretty `header` and
-  `diff_lines` are lazy properties, built only if accessed.
+  chain, and version info for migration events.  The default `"aborted"`
+  rendering is `<action>[ by <user>] transaction aborted: <error>` with the
+  action and user colored and the user resolved via `logfmt`.  Pretty
+  `header` and `diff_lines` are lazy properties, built only if accessed.
 - `@kanta.logemit` registers a callback receiving the event.  The callback
   decides what is logged and where: it may log one or more messages on
   `event.logger`, log somewhere else, or nothing at all.  A falsy return