test: web worker coverage — supabase.db + supabase.websocket

[h0lybyte]

Tracker: #11592

Problem

`apps/kbve/astro-kbve/src/workers/supabase.db.ts` and `supabase.websocket.ts` proxy Supabase RPC + realtime through a Web Worker so the main thread stays responsive. There are 0 tests. The existing `astro check` flagged unused `WorkerResponse` types in both files — a sign that internals aren't being exercised.

Acceptance

• Stand up a vitest project that can import the worker source directly (no Worker constructor needed; the worker code is just a module)
• `supabase.db.test.ts`:
  • Message types in / out are exhaustive (insert / update / upsert / delete / select / RPC)
  • Errors from `@kbve/droid` strategies surface as `{ ok: false, error }` responses, never thrown
  • Race conditions: two simultaneous RPC calls with the same id resolve independently
• `supabase.websocket.test.ts`:
  • Channel subscribe / unsubscribe lifecycle
  • The `await channel.subscribe()` warning (currently flagged as no-effect) is either fixed or explicitly justified
  • Backoff on reconnect doesn't tight-loop when the server is down