Document kanta.ctx as the app-context channel for logemit

kanta.ctx already existed as a user-writable namespace for migrations;
since LogEvent carries the kanta instance, ev.kanta.ctx is also the way
for applications to pass per-connection metadata to their logemit
callbacks, including for creation/bootstrap events. No API change needed.

docs/database.md

@@ -235,6 +235,9 @@ def resolve_user_key(value: str) -> str | None:
   `kanta` instance, 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.
+  Application-specific context (e.g. a connection id) can be stored in
+  `kanta.ctx` — a user-writable namespace — and read back in callbacks as
+  `event.kanta.ctx`, which also covers creation/bootstrap events.
 - The built-in formatting is assembled from standard blocks that custom
   emitters can reuse as-is or replace piecemeal:
   - `event.header` — a lazy property producing the default one-line header