Adds the two starter monster definitions and their location-monster
mapping (weights 70/30) to the vertical-slice seed, following the
same findOneBy/update-or-insert pattern used for locations, plus an
upsert on (locationId, monsterId) for the mapping. Copies the source
artwork into the served images/monsters directory and extends the
seed spec harness to cover both idempotent inserts.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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