POST /c/{id}/fingerprint handler that decodes a JSON fingerprint body
and merges it into the most recent event's Extra JSONB for the same
(token_id, source_ip) tuple within a 30s window via the existing
event.Repository.AttachFingerprint method. Decoupled from the concrete
repository through a small FingerprintAttacher interface so future
event.Service can swap in without touching this package.
Per spec §9.2 the fingerprint POST is enrichment-only: a missing match
(event.ErrNotFound), an invalid JSON body, an empty body, and a body
exceeding 64 KiB all silently return 204. Only unexpected repository
errors are logged via slog. The token id is read from chi.URLParam("id")
so the route mounts naturally under the existing trigger router in
Phase 9.
Three integration tests against testcontainers cover the happy-path
JSONB merge, the no-matching-event path (silent 204), and the
invalid-JSON path (204 plus stored Extra left untouched).
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