fix(oauth): clear the token-refresh backoff when a downstream token is saved

[pedrofrxncx]

Bug in apps/api/src/oauth/token-refresh.ts / apps/api/src/api/routes/downstream-token.ts.

refreshAndStore arms a per-connection exponential backoff (refreshBackoff, up to REFRESH_BACKOFF_CAP_MS = 5 minutes) after a failed refresh, so a broken upstream isn't hammered. That map is keyed only by connectionId and, until now, was only ever cleared on a successful refresh or on connection delete (#6980).

Failure scenario: a connection's token refresh keeps failing (upstream blip, or a refresh_token that's dead but not classified permanent) and arms the backoff window. The user then reconnects — the frontend calls POST /connections/:id/oauth-token with a brand-new, never-used access+refresh token. If a proxy request needs a forced refresh (e.g. it just got a 401) before the old backoff window naturally expires, refreshAndStore short-circuits on the stale backoff entry and returns null immediately, without even attempting the new refresh_token. The connection looks broken for up to 5 minutes after the user did everything right to fix it.

Fix: call clearRefreshBackoff(connectionId) right after the token upsert in the oauth-token POST route — a freshly saved token has never failed, so any suppression window is stale.

Regression test: added clears the token-refresh backoff entry on save to downstream-token.integration.test.ts, mocking ../../oauth/token-refresh (same mock.module pattern as delete.test.ts's equivalent case for #6980) and asserting clearRefreshBackoff is called with the connection id after a successful save.

To confirm: bun test apps/api/src/api/routes/downstream-token.integration.test.ts (needs a running Postgres — this laptop has none, so I verified with cd apps/api && bunx tsc --noEmit (clean) and bunx oxlint on both changed files (0 warnings/errors); full CI runs the integration suite.

Does not touch delete.ts/delete.test.ts (already covered by open PR #6980) — this is the reconnect/save path, not the delete path.

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Summary by cubic

Clears the token-refresh backoff when a downstream token is saved, so reconnects aren't blocked by a stale suppression window from the previous dead token.

• Previously, a failed refresh armed a per-connection backoff that could bounce forced refreshes for up to 5 minutes even after a fresh token was saved.
• The POST /connections/:id/oauth-token route now calls clearRefreshBackoff immediately after the token upsert.
• Adds a regression test that asserts the backoff is cleared on save.

^{Written for commit 568eb01. Summary will update on new commits.}