docs(plan): add M4a outbound scrobble worker implementation plan
12-task TDD-shaped plan covering: migration 0008 (users LB columns +
scrobble_queue table), sqlc query additions, pure ListenBrainz HTTP
client with typed errors and httptest coverage, threshold function,
MaybeEnqueue with idempotent insert + threshold gate, backoff schedule
1m→5m→30m→2h→6h, Worker with batched submit + retry/fail/auth handling,
playevents.Writer post-commit best-effort hooks, GET/PUT
/api/me/listenbrainz endpoints, /settings page in SvelteKit, full
verification sequence.
bvandeusen
a90deab862