Fast-path YouTube destination connect: reuse an existing active YoutubeConnection's tokens (refresh if near expiry) instead of a fresh Google round-trip, and commit the Destination change directly.

POST-only. This path writes to the DB on the caller's say-so alone (no Google round-trip to re-confirm consent), so it must not be reachable by a plain GET: SameSite=Lax still attaches the session cookie to a top-level cross-site GET navigation, so a GET version of this fast path could be triggered by a bare link. POST closes that, matching the app's mutations-aren't-GETs rule. Returns 204 when there's no active connection to reuse; the caller falls back to the full-OAuth GET sibling below.