feat: XState worker rewrite + portless stability

Summary

• Migrate backend worker orchestration to an XState actor architecture (supervisor/playlist/song runtime) while preserving existing external HTTP and WebSocket APIs.
• Replace queue/runtime internals with actor-style behavior for clearer scheduling, better cancellation, and safer diagnostics.
• Improve portless/local environment stability for cover assets and API URL handling.
• Fix runtime regressions in song actor handoff and queue cancellation paths.

What's New

• Worker runtime now uses a unified actor-driven lifecycle for songs and playlists.
• Queue behavior is aligned with actor model expectations, including cancellation and retry-safe transitions.
• /covers proxy in frontend dev config now uses the active API target from runtime config.
• VITE_API_URL handling in portless web startup now supports both env-file and runtime env values.
• Song actor cancellation path now avoids stale actor stop races when run() is re-entered.
• Diagnostics event pipeline includes tighter runtime lifecycle handling.

Detailed Changes

• apps/server/src/worker/runtime/actor-runtime.ts: event-loop and snapshot bookkeeping refinements for safer supervisor behavior.
• apps/server/src/worker/song-worker.ts: safer song actor handoff and cancellation semantics.
• apps/web/vite.config.ts: host-aware /covers proxy target and robust env fallback.
• apps/server/src/index.ts, package.json, pnpm-lock.yaml, AGENTS.md, README.md: supporting runtime/config/docs updates.

Validation

• pnpm check
• pnpm -r test
• pnpm -r typecheck
• pnpm react:doctor