Send msg id to courier fetch-attachments endpoint

[rowanseymour]

and clean up flows.MsgID vs models.MsgID

[codecov]

Codecov Report

Merging #688 (3a02cfe) into main (fd9b784) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #688      +/-   ##
==========================================
- Coverage   62.18%   62.17%   -0.01%     
==========================================
  Files         132      132              
  Lines       12867    12868       +1     
==========================================
  Hits         8001     8001              
- Misses       4007     4008       +1     
  Partials      859      859              

Impacted Files	Coverage Δ
core/models/msgs.go	56.69% <0.00%> (ø)
core/msgio/courier.go	66.66% <0.00%> (-0.91%)	⬇️
core/tasks/handler/worker.go	51.90% <66.66%> (ø)
core/models/sessions.go	54.81% <100.00%> (ø)

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