Route transaction aborts through logemit as 'aborted' events

The rollback warning is now a LogEvent (kind='aborted', WARNING level,
carrying the exception) dispatched via emit_event so logemit callbacks can
handle or restyle it. Default rendering: action in transaction color
without quotes, followed by ' transaction aborted: {exc}' in default
color.

docs/database.md

@@ -225,16 +225,17 @@ def resolve_user_key(value: str) -> str | None:
 #### Log Emitters
 
 - Every change-related message Kanta emits (transaction/bootstrap/migration
-  changes, `Created <file>`, migration summaries) is described by a
-  `kanta.logging.LogEvent` and dispatched through `kanta.logging.emit_event`.
-  Kanta's own output goes through the same mechanism: when no `logemit`
-  callback handles an event, `kanta.logging.default_emit` renders it with the
-  built-in formatting.
+  changes, `Created <file>`, migration summaries, aborted transactions) is
+  described by a `kanta.logging.LogEvent` and dispatched through
+  `kanta.logging.emit_event`.  Kanta's own output goes through the same
+  mechanism: when no `logemit` callback handles an event,
+  `kanta.logging.default_emit` renders it with the built-in formatting.
 - A `LogEvent` carries the event `kind` (`"change"`, `"created"`,
-  `"migrated"`), the preferred `logger` and `level`, and all relevant state:
-  `action`, `user`, `extra`, `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.
+  `"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.
 - `@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
@@ -242,8 +243,8 @@ def resolve_user_key(value: str) -> str | None:
   passes the event — possibly modified — to the next registered callback.
   When all callbacks pass, `default_emit` renders the event; a callback may
   also call `default_emit(event)` itself to delegate events it does not
-  customize.  Operational diagnostics (rollback warnings, integrity errors)
-  do not go through this mechanism.
+  customize.  Operational diagnostics (integrity errors, background flush
+  failures) do not go through this mechanism.
 - Logging never breaks functionality: a crashing `logemit` callback is
   reported with `logger.exception` and the event falls back to the built-in
   formatting; if the built-in formatting itself fails, the error is reported