Plex: repair a rejected server token or moved address instead of signing the person out

The saved Plex server token or address can stop working (token rotated, server moved). That
looked like "signed out" and sent the person back to the sign-in screen. The proxy now asks plex.tv
(using the saved account sign-in) for a fresh token and address, retries once, and saves them.
Sign-ins saved before the server id was recorded are matched by server name. A revoked account
sign-in signs the person out cleanly; repairs are one at a time and pause 30s after a failure.
The server logs [plex] lines for sign-in, connect, repair and failure.

tools/plex-test.js (31 checks, fake plex.tv + fake Plex) runs in the Gitea workflow.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -19,6 +19,7 @@ jobs:
       - run: npm ci
       - run: node tools/smoke-test.js
       - run: node tools/users-test.js   # accounts: private data, tab limits, invites, migration
+      - run: node tools/plex-test.js    # Plex sign-in stays signed in and repairs itself
 
       - uses: docker/setup-buildx-action@v3