Fix GitHub App auth: string JWT issuer (PyJWT >=2.10)

[jontyms]

Problem

The prod sync crashes on startup when authenticating as the GitHub App:

TypeError: Issuer (iss) must be a string.
  github3/apps.py create_token -> jwt.encode(... "iss": app_id ...)

core.py stored GITHUBAPP_ID as an int, and github3.py 4.0.1 uses that value directly as the JWT iss claim. PyJWT >= 2.10 (we resolve 2.13.0) now rejects non-string issuers, so token minting fails and every sync errors out.

Fix

Store the app id as a string (still validated as numeric via str(int(...))). github3's create_token uses app_id only as iss, so this is safe, and the issuer claim is now valid.

Chosen over pinning PyJWT<2.10 so we don't hold back a security-relevant dependency.

Verification

Locally, apps.create_token(key, 2156025) raises the TypeError; apps.create_token(key, "2156025") returns a valid token.

Merging this rebuilds ghcr.io/hackucf/github-team-sync:latest; the prod Deployment then needs a rollout restart to pull it.

🤖 Generated with Claude Code

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.