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When multiple goroutines/workers concurrently refresh the same OAuth token,
the first succeeds but invalidates the old refresh_token (rotation). Subsequent
attempts using the stale token get invalid_grant, which was incorrectly treated
as non-retryable, permanently marking the account as ERROR.

Three complementary fixes:
1. Race-aware recovery: after invalid_grant, re-read DB to check if another
   worker already refreshed (refresh_token changed) — return success instead
   of error
2. In-process mutex (sync.Map of per-account locks): prevents concurrent
   refreshes within the same process, complementing the Redis distributed lock
3. Increase default lock TTL from 30s to 60s to reduce TTL-expiry races
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Introduce OAuthRefreshAPI as the single entry point for all OAuth token
refresh operations, eliminating the race condition where background
refresh and inline refresh could simultaneously use the same
refresh_token (fixes #1035).

Key changes:
- Add OAuthRefreshExecutor interface extending TokenRefresher with CacheKey
- Add OAuthRefreshAPI.RefreshIfNeeded with lock → DB re-read → double-check flow
- Add ProviderRefreshPolicy / BackgroundRefreshPolicy strategy types
- Simplify all 4 TokenProviders to delegate to OAuthRefreshAPI
- Rewrite TokenRefreshService.refreshWithRetry to use unified API path
- Add MergeCredentials and BuildClaudeAccountCredentials helpers
- Add 40 unit tests covering all new and modified code paths