Refactor: use separate apis for polling and attachment handling

[max-nextcloud]

• chore(types): type polling backend and notify service
• refactor(connection): use sync api instead of SessionConnection
• chore(refactor): use api functions for attachment handling

[codecov]

Codecov Report

Attention: Patch coverage is 73.29193% with 43 lines in your changes missing coverage. Please review.

Project coverage is 59.61%. Comparing base (57d8424) to head (889dd69).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/services/PollingBackend.ts	42.10%	22 Missing ⚠️
src/apis/sync.ts	5.26%	18 Missing ⚠️
src/services/SyncService.ts	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7446      +/-   ##
==========================================
+ Coverage   59.52%   59.61%   +0.09%     
==========================================
  Files         493      494       +1     
  Lines       37378    37376       -2     
  Branches     1087     1088       +1     
==========================================
+ Hits        22248    22282      +34     
+ Misses      15024    14988      -36     
  Partials      106      106              

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[max-nextcloud]

superseeds #7442

[mejo-]

Great work, thanks for the continuous cleanup efforts ❤️