sources/telegram: implement connecting existing user to a Telegram account

[atereshkin]

Details

This PR implements missing "connect account" functionality for the Telegram source. When a Telegram source is configured, existing users should be able connect their Telegram account by going to Settings -> Connected services. Currently this functionality is missing and this PR addresses that.

A respective view is added in sources/telegram/api/source.py and renderConnectButton is implemented in SourceSettingsTelegram.ts.

This closes #18467

Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• [n/a] The documentation has been updated
• [n/a] The documentation has been formatted (make docs)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.11%. Comparing base (eb60276) to head (4341fe4).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18517      +/-   ##
==========================================
+ Coverage   93.02%   93.11%   +0.08%     
==========================================
  Files         911      931      +20     
  Lines       49848    50880    +1032     
==========================================
+ Hits        46373    47375    +1002     
- Misses       3475     3505      +30     

Flag	Coverage Δ
e2e	44.84% <50.68%> (-0.41%)	⬇️
integration	22.73% <1.36%> (-0.15%)	⬇️
unit	91.37% <100.00%> (+0.16%)	⬆️
unit-migrate	91.41% <100.00%> (+0.16%)	⬆️

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