WAN-97: tighten self-host startup readiness

[haoxianhan]

Summary

• Unifies self-host backend port derivation around BACKEND_PORT, keeping API_PORT/SERVER_PORT/PORT only as compatibility aliases.
• Makes start.sh server-stack startup by default, waits on backend /readyz plus frontend /, fails fast with targeted diagnostics, and makes source daemon startup explicit opt-in via SELFHOST_START_DAEMON=true.
• Updates Makefile/install/docs paths so localhost, specific LAN IP, and reverse proxy modes no longer conflict with CLI/daemon setup boundaries.

Review notes

• This draft publishes the already-verified WAN-97 local branch through the fork path.
• The PR currently includes two commits because the WAN-97 worktree was based on an existing local prerequisite commit 25c039625 (my script) that added start.sh/stop.sh; the WAN-97 cleanup commit is 549ac5487.
• No Docker pull/up, LAN/public smoke, production, cluster, data, or secret operation was performed for this publication step.

Verification

• bash scripts/selfhost-config.test.sh
• bash -n start.sh scripts/selfhost-env.sh scripts/local-env.sh scripts/install.sh scripts/selfhost-config.test.sh
• git diff --check
• docker compose --env-file .env.example -f docker-compose.selfhost.yml config
• BACKEND_PORT=9100 FRONTEND_PORT=3100 docker compose --env-file .env.example -f docker-compose.selfhost.yml config
• make -n selfhost-wait
• BACKEND_PORT=9100 FRONTEND_PORT=3100 make -n selfhost-wait
• BIND_HOST=192.168.1.100 BACKEND_PORT=9100 FRONTEND_PORT=3100 make -n selfhost-wait
• go test ./cmd/multica -run 'TestResolveSelfHostServerURL|TestSelfHostAppURLHonorsEnv|TestPersistSelfHostConfigIfReachable'

[vercel]

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