fix: resolve error on multi-attendee invoice downloads

[abhinavk96]

Fixes #6109

Changes proposed in this pull request:

• Fixes flawed attendee access control logic
• allows recreation of invoice file, in case it does not exist, otherwise, it just crashed.

[codecov]

Codecov Report

Merging #6110 into development will decrease coverage by <.01%.
The diff coverage is 10%.

@@               Coverage Diff               @@
##           development    #6110      +/-   ##
===============================================
- Coverage         66.2%   66.19%   -0.01%     
===============================================
  Files              285      285              
  Lines            14197    14198       +1     
===============================================
  Hits              9399     9399              
- Misses            4798     4799       +1

Impacted Files	Coverage Δ
app/api/auth.py	24.12% <10%> (-0.11%)	⬇️

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[codecov]

Codecov Report

Merging #6110 into development will decrease coverage by <.01%.
The diff coverage is 10%.

@@               Coverage Diff               @@
##           development    #6110      +/-   ##
===============================================
- Coverage         66.2%   66.19%   -0.01%     
===============================================
  Files              285      285              
  Lines            14197    14198       +1     
===============================================
  Hits              9399     9399              
- Misses            4798     4799       +1

Impacted Files	Coverage Δ
app/api/auth.py	24.12% <10%> (-0.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 88d1b13...89acb2b. Read the comment docs.