fix(geschichte): make the documented error-code contract real

GESCHICHTE_TYPE_IMMUTABLE and JOURNEY_NOTE_TOO_LONG were declared in
errors.ts, translated, and documented — but never existed in the backend.
update() now rejects a type change with 409 (omitted/same type still pass);
note length is enforced at 2000 with its own code, matching the frontend
maxlength and the i18n message (resolves the #793 discrepancy in favour of
the spec). JOURNEY_ITEM_NOT_IN_JOURNEY is deleted everywhere instead — the
deliberate 404 posture for cross-journey item ids must not leak existence
via a distinct code.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/messages/en.json

@@ -1203,7 +1203,6 @@
 	"journey_edit_title_journey": "Edit reading journey",
 	"journey_publish_disabled_title": "Title and at least one entry required",
 	"journey_save_hint_published": "Changes will be immediately visible to all readers.",
-	"error_journey_item_not_in_journey": "This entry does not belong to this reading journey.",
 	"error_journey_note_too_long": "The note is too long (maximum 2000 characters).",
 	"error_journey_document_already_added": "This letter is already included in the reading journey.",
 	"error_geschichte_type_immutable": "The type of a story cannot be changed after creation."