fix(auth): set supplementalVideoIds on first-login channel creation

upsertGoogleUser's nested channels.create (the auto-provisioning path
for a brand new team's first Google login) never set
supplementalVideoIds, a required String[] column with no DB default
(dropped intentionally in 20260608000002 to match the Prisma schema).
channel-import.service.ts already sets it explicitly on its create path
- this path was just missed, and nothing had exercised a real first
login against Postgres until now.

backend/src/modules/auth/auth.service.ts

@@ -67,6 +67,7 @@ export class AuthService {
               youtubeChannelId,
               name: channelName,
               uploadsPlaylistId,
+              supplementalVideoIds: [],
               youtubeAccessToken: this.encrypt(accessToken),
               youtubeRefreshToken: refreshToken ? this.encrypt(refreshToken) : undefined,
               youtubeTokenExpiry: new Date(Date.now() + 3600 * 1000),