core: refactor account module to Effect with repo/service split

Rewrite the account layer using Effect for typed error handling, schema
validation, and structured concurrency. Split into three concerns:

- schema.ts: branded types (AccountID, OrgID, AccessToken) and shared
  data classes
- repo.ts: AccountRepo service owning all DB operations via drizzle
- service.ts: AccountService owning HTTP orchestration (OAuth device
  flow, token refresh, org/config fetching) with AccountRepo as a
  dependency

Key improvements:
- Branded types enforce type safety at API boundaries
- Option used consistently instead of null/undefined in internal APIs
- User and orgs fetches parallelized during login poll
- Schema-validated HTTP request/response bodies
- Transient read retry with exponential backoff
- Clock.currentTimeMillis instead of Date.now() for testability
- 11 new repo tests covering all DB operations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>