The CharacterItem idempotency check was looking up by the seed's own
literal id instead of the (characterId, itemDefinitionId) unique index
that CharacterItem actually enforces. If the demo character had already
looted a worn-short-sword naturally, re-running the seed would miss
that row and try to insert a colliding duplicate, breaking seeding
instead of being a safe no-op. Look up by the real domain key and reuse
whatever id is found when wiring up the CharacterEquipment row.
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