feat(nakama): add BodyTime event flow

[JOY (JOY)]

Summary

• Add secondspawn_bodytime_event to the Nakama runtime for prototype BodyTime earn, spend, and danger-zone drain events.
• Validate event kind/source pairs, cap per-event amounts, enforce retry idempotency when an event ID is supplied, and add a short prototype cooldown for repeated earn claims from the same source.
• Log accepted BodyTime events into bounded agent_activity, expose body lifecycle in Unity DTOs, and add a Unity gateway wrapper for the Nakama RPC.
• Mark bodies as dead when BodyTime reaches zero; further non-idempotent BodyTime mutations are rejected until reincarnation flow exists.
• Update the pre-alpha changelog.

Verification

• npm run build in backend/nakama
• npm test in backend/nakama
• go test -count=1 ./... in backend/gateway
• go vet ./... in backend/gateway
• npx --yes markdownlint-cli2@0.18.1 "*.md" "docs/**/*.md" ".github/**/*.md"
• git diff --check
• Changed-file em-dash/en-dash scan

Local Review

Local code-review skill pass result: approved with suggestions. No engine specialist is configured in .Codex/docs/technical-preferences.md, so specialist routing was skipped. The implementation follows ADR 0010 by keeping game-backend profile/economy state in Nakama runtime modules, and follows the Time-as-Currency rule that BodyTime mutations are server-authoritative validated intents.

Caveat

Unity clean-worktree batchmode compile is still blocked by the existing Package Manager path undefined issue in this repo family, so Editor/MCP compile verification remains required before a no-caveat Unity merge claim.

Gemini (@gemini-code-assist) review
Codex (@codex) review

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[JOY (JOY)]

Local delta review after rebase onto dev:

• Verified BodyTime RPC coexists with the actor profile registry from dev.
• Checked source caps, retry idempotency, earn cooldown, activity logging, and zero-time death behavior in backend/nakama/tests/supabase_custom_auth.test.mjs.
• Verified Unity DTO additions are limited to BodyTime/lifecycle surfaces and do not grant authority.
• Reviewer bots are quota-limited on this PR, so this is the local code-review fallback.

Verification:

• npm run build && npm test in backend/nakama
• go test -count=1 ./... && go vet ./... in backend/gateway
• markdownlint-cli2
• git diff --check

Verdict: APPROVED for merge into dev.