Add example docker-compose for manager, frontend and SMB

[birme]

Summary

• Add a single self-contained docker-compose.yml at the repo root that brings up the full Open Intercom stack on one host: manager, frontend, sfu (Symphony Media Bridge) and mongo.
• Wire sensible defaults so docker compose up works out of the box: manager points at SMB_ADDRESS=http://sfu:8180 with SMB_APIKEY matching the SFU API_KEY (default eyevinn), DB_CONNECTION_STRING=mongodb://mongo:27017/intercom-manager, CORS_ORIGIN=http://localhost:3000, and PUBLIC_HOST=http://localhost:8000.
• Frontend bakes MANAGER_URL into the web build at container start and publishes port 3000; manager on 8000; SFU exposes its HTTP API on 8180 and UDP 10000 media.
• MongoDB uses a named volume and a healthcheck, with the manager depends_on mongo service_healthy.
• Inline comments document production notes, including that the SFU typically needs host networking for RTP.

Test plan

This is a docs/deployment-example change only — a single new docker-compose.yml is added and no application code changed, so the standard npm test, npm run typecheck, and npm run lint are unaffected. The meaningful verification is at the compose level:

• docker compose config validates the file (no schema/interpolation errors)
• docker compose up brings the stack up and the UI is reachable at http://localhost:3000
• Manager reaches the SFU and MongoDB with the default env wiring

Closes #152

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[birme]

Code Review — Verdict: LGTM

Self-authored PR reviewed by daily-backlog-pr Phase 3 (code-reviewer invocation, separate from the implementer). Since GitHub blocks state-bearing self-review by the author's own account, the verdict is recorded here as a marker instead of an --approve review.

Adds only docker-compose.yml (85 lines, no application code). Every service's env wiring was verified against the actual upstream images and this repo's config: manager env vars match src/config/load-env.ts/.env.example; SFU API_KEY/ports/cap_add: SYS_NICE/ulimits rtprio:99 match Eyevinn/docker-wrtc-sfu; frontend PORT/MANAGER_URL (browser-reachable localhost) correct; mongo healthcheck + depends_on: service_healthy gating correct. No hardcoded secret (the eyevinn SMB_APIKEY default is the upstream-documented, overridable, host-internal inter-service key), no yarn, no rm -rf, no forced push.

Suggestions only (non-blocking): add an inline "override SMB_APIKEY in production" note, mention NUM_UDP_PORTS for scaled setups, and link the file from the README.

No Blocking items — LGTM. This PR now awaits a human with a different account to gh pr review --approve and merge (the automation cannot self-approve).