Adds startHunt's server-authoritative flow: complete due travel, verify
the location allows hunting, weighted-random-roll exactly 3 encounters
from the location's enabled monster pool inside a locked transaction
that supersedes any prior active hunt, and compute a danger rating per
encounter from the rolled monster's own stats. Mirrors TravelService's
transaction/locking pattern and error conventions.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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