chore: docker-compose.yml (app + frontend + jaeger)

[constk]

Problem

Local dev should mirror prod topology: backend, frontend, and OTel collector all up with one command.

Proposed solution

Port docker-compose.yml: app service (built from Dockerfile, port 8000, env vars for OTel exporter endpoint), frontend service (Vite dev server in dev mode, port 5173), jaeger service (jaegertracing/all-in-one, ports 16686 UI + 4317 OTLP). Networks linked. Volumes for hot-reload during dev.

Acceptance criteria

• docker compose up brings all three services up; UI reachable at http://localhost:5173 (after feat: frontend scaffold (Vite + React 19.2 + TS strict, eslint flat + prettier + vitest) #21-feat: hello page hitting /api/v1/health + CSS-variable palette + sample component test #23).
• Backend /api/v1/health reachable at http://localhost:8000 (after feat: backend scaffold (FastAPI app, /api/v1/health, /api/v1/echo, sessions) #17).
• Jaeger UI reachable at http://localhost:16686.
• OTLP traces from app appear in Jaeger UI (verified end-to-end in test: end-to-end smoke (docker compose up, sample PR runs every gate green, eval workflow_dispatch) #27).

Priority rationale

High: the three-service compose IS the dev experience — one command to a working stack with traces.

Depends on

#6

[constk]

Auto-close did not fire from the squash-merge body. Marking closed retrospectively — the work shipped weeks ago and the project board's Done column reflects the actual state once the project-item move that follows this comment lands.