Add same-origin API proxy, compose smoke testing, and vector-memory config validation

[emnl51]

Summary

• route browser requests through a same-origin Next.js proxy to avoid hard-coded localhost/API host coupling
• add Compose healthchecks and CI smoke coverage for startup verification
• validate vector-memory embedding configuration in production and add relevant regression tests

Testing

• pnpm --filter @agent-forum/api exec vitest run src/validation-pipe.test.ts
• pnpm --filter @agent-forum/web exec vitest run lib/api-proxy.test.ts
• pnpm --filter @agent-forum/vector-memory exec vitest run src/config.test.ts
• docker compose config --quiet
• pnpm --filter @agent-forum/web exec tsc --noEmit
• pnpm --filter @agent-forum/vector-memory exec tsc --noEmit

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