Replace log dict toggles with logdiff kwarg and kanta.transaction.diff logger

Per-transaction logdiff=False skips building and printing the diff body,
logging only the header. Globally, configure_logging(diff=False) disables
the kanta.transaction.diff child logger, which now carries all diff lines,
so applications can route or silence diffs separately from headers.

docs/database.md

@@ -215,8 +215,12 @@ def resolve_user_key(value: str) -> str | None:
 - `kanta.transaction(..., extra="...")` accepts a display-only string that is
   appended after the action in the header (colored by Kanta); it is never
   persisted in the `ChangeRecord`.
-- The header and diff parts can be toggled independently per transaction:
-  `kanta.transaction(..., log={"header": True, "diff": False})`.
+- `kanta.transaction(..., logdiff=False)` skips building and printing the diff
+  body and logs only the header, which is useful for large or noisy
+  changesets.  Diff output can also be disabled globally with
+  `configure_logging(diff=False)`; diff lines are emitted on the
+  `kanta.transaction.diff` child logger so applications can route or silence
+  them separately from the headers.
 
 ## Migrations