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

kanta/kanta.py

@@ -130,7 +130,10 @@ class Kanta(Generic[T]):
         """User-writable context namespace.
 
         Migration functions receive the ``Kanta`` instance and can read or
-        mutate ``kanta.ctx`` during migrations.
+        mutate ``kanta.ctx`` during migrations.  Applications can also store
+        arbitrary data here (e.g. a connection id); since
+        :class:`kanta.logging.LogEvent` carries the Kanta instance, logemit
+        callbacks can read it as ``event.kanta.ctx``.
         """
         return self._impl.ctx
 

tests/test_logemit.py

@@ -344,3 +344,18 @@ def test_header_is_settable_and_used_by_default_emit(capsys):
     err = capsys.readouterr().err
     assert "CUSTOM update" in err
     assert "counter" in err  # default diff routing still applies
+
+
+@pytest.mark.asyncio
+async def test_ctx_reachable_from_event(tmp_path, format_config):
+    path = tmp_path / "test.db"
+    kanta = make_kanta(path, Data, format_config)
+    kanta.ctx.connection_id = 7
+    seen = []
+    kanta.logemit(lambda ev: seen.append(ev.kanta.ctx.connection_id) or True)
+    await kanta.open()
+    with kanta.transaction(action="inc") as data:
+        data.counter = 1
+    await kanta.close()
+
+    assert seen and all(connection_id == 7 for connection_id in seen)