[second2falcon]

Problem

An expired or invalid session token in twitch_oauth.txt makes streamlink fail with a generic No playable streams found. That looks the same as the channel being offline, so the real cause is hidden.

Changes

• TwitchHelper.getSessionToken() reads the token and checks it against id.twitch.tv/oauth2/validate. It doesn't touch the app token (accessToken).
  • 401: logs an error, sends a system notification and returns undefined. Capture continues without auth (no ad-skip or sub-only access) instead of failing.
  • Network or other errors: the token is kept, so a Twitch hiccup doesn't drop auth.
  • The token is added to censoredLogWords here, replacing the per-call-site censoring.
• Live capture: a TwitchAutomator.captureVideo() override validates the token before streamlink starts, because providerArgs() is sync.
• VOD-at-end download: uses the same helper. A missing file still throws when auth_enabled is on.
• Scheduler: checks the token every 6 hours so an expired one is reported before the next stream.
• Translations: new notify.twitch-oauth-token-expired-or-invalid-check-config-twitch-oauth-txt key, English only.

Not included

• Detecting the auth failure in streamlink's output inside captureTicker. There's no confirmed error string yet.
• Showing the token's status in the UI.

Testing

Not typechecked or run locally because dependencies weren't installed. To verify: put a bad token in twitch_oauth.txt, start a capture, and expect a "Twitch token invalid" notification with the recording still going.

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