OpenGraph: use repo avatar if exist

[6543]

as title

come up in https://codeberg.org/Codeberg/Community/issues/237

[codecov-commenter]

Codecov Report

Merging #12586 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12586      +/-   ##
==========================================
+ Coverage   43.39%   43.40%   +0.01%     
==========================================
  Files         645      645              
  Lines       71281    71283       +2     
==========================================
+ Hits        30931    30940       +9     
+ Misses      35335    35327       -8     
- Partials     5015     5016       +1     

Impacted Files	Coverage Δ
models/repo.go	55.69% <100.00%> (+0.06%)	⬆️
modules/indexer/stats/queue.go	64.70% <0.00%> (-11.77%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
services/pull/pull.go	42.03% <0.00%> (+0.46%)	⬆️
routers/repo/view.go	37.98% <0.00%> (+0.64%)	⬆️
modules/log/event.go	57.54% <0.00%> (+0.94%)	⬆️
modules/queue/unique_queue_disk_channel.go	55.38% <0.00%> (+1.53%)	⬆️
models/unit.go	49.31% <0.00%> (+2.73%)	⬆️

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