Demo: plain-text custom header colored by Kanta, rename migration, str extras

docs/database.md

@@ -212,33 +212,28 @@ def resolve_user_key(value: str) -> str | None:
 
 - By default a transaction is logged with an `action by user` header followed
   by the diff lines.  Added paths are colored green, deleted paths red.
-- `kanta.transaction(..., extra=...)` accepts display-only metadata that is
-  used for logging and is never persisted in the `ChangeRecord`:
-  - a string is appended literally after the action in the default header,
-  - a dict is passed to a registered `@kanta.logheader` callback; if it has
-    no `"target"` key, the database filename is inserted as the target.
-- Register a `@kanta.logheader` callback to replace the entire header line.
-  It may declare `action: str`, `user: str | None` and `extra: dict | None`
-  parameters, and can also have `DictPre`/`DictPost` state dicts and the
-  `Kanta` instance injected.  It must be synchronous and return `str | None`.
+- `kanta.transaction(..., extra="...")` accepts a display-only string that is
+  appended after the action in the default header (colored by Kanta); it is
+  never persisted in the `ChangeRecord`.
+- Register a `@kanta.logheader` callback to compose a custom header.  Declare
+  any of `action: str`, `user: str`, `extra: str`: Kanta passes the parts
+  with its header colors already applied (missing `user`/`extra` as empty
+  strings), so callbacks only arrange text — no color codes, fallbacks, or
+  padding.  `DictPre`/`DictPost` state dicts and the `Kanta` instance can
+  also be injected.  The callback must be synchronous and return `str | None`.
 - Multiple logheader callbacks are stacked in registration order; the first
   callback to return a non-`None` result wins.  If all return `None`, Kanta
-  falls back to the default header.  The `user` value passed to the callback
-  has already been through the `logfmt` formatters.
+  falls back to the default header.  The `user` part has already been through
+  the `logfmt` formatters.
 - The header and diff parts can be toggled independently per transaction:
   `kanta.transaction(..., log={"header": True, "diff": False})`.
 
 ```python
 @kanta.logheader
-def format_header(action: str, user: str | None, extra: dict | None) -> str:
-    session = extra.get("session_id", "-")
-    return f"{user:<20} {session:>2}  {action}  {extra['target']}"
+def format_header(action: str, user: str, extra: str) -> str:
+    return f"{user}  {action}  {extra}"
 
-with kanta.transaction(
-    action="update",
-    user="alice",
-    extra={"session_id": 3, "target": "Project Name (abcd1234)"},
-) as data:
+with kanta.transaction(action="update", user="alice", extra="Project X") as data:
     ...
 ```