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>
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