feat(geschichte): allow journey items on STORY-type Geschichten (#795)

Delete the JOURNEY-only type guard in JourneyItemService.append() so the
existing item endpoints serve both Geschichte types. GeschichteType has
exactly two constants, so an allowlist replacement would be unreachable
dead code. Fix the not-found messages that claimed "Journey", and remove
the now-orphaned GESCHICHTE_TYPE_MISMATCH error code end to end
(ErrorCode, errors.ts union + mapping, i18n keys in de/en/es).

Tests: three STORY append unit tests written red against the guard, plus
end-to-end STORY coverage (append+retrieve, V72-style position-gap rows
incl. removal, dangling document-deleted item). The two STORY-rejection
tests die with the guard — no third enum value exists to feed it.

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

frontend/messages/en.json

@@ -1028,7 +1028,6 @@
 	"error_journey_item_not_found": "The journey item was not found.",
 	"error_journey_item_position_conflict": "The order was just changed by someone else — please reload the page.",
 	"error_journey_at_capacity": "The reading journey has already reached the maximum of 100 items.",
-	"error_geschichte_type_mismatch": "This story is not a reading journey — journey items are not allowed here.",
 	"journey_item_document_deleted": "[Document deleted]",
 	"geschichten_index_title": "Stories",
 	"geschichten_new_button": "New story",