Removed dependency on redundant session_answers table + optimized some mongo queries

[deepansh96]

Fixes #83 #82
ADR - https://www.notion.so/avantifellows/ADR-Quiz-Backend-DB-optimization-66e73d0531854098a2702fee070e2fc5

Summary

[pritamps]

Looks good to me. Just questions for my own context building!

[codecov]

Codecov Report

Patch coverage: 81.25% and project coverage change: -1.07 ⚠️

Comparison is base (f970b54) 95.55% compared to head (35ca665) 94.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   95.55%   94.48%   -1.07%     
==========================================
  Files          16       16              
  Lines         697      707      +10     
==========================================
+ Hits          666      668       +2     
- Misses         31       39       +8     

Impacted Files	Coverage Δ
app/models.py	100.00% <ø> (ø)
app/routers/sessions.py	84.15% <72.72%> (-4.98%)	⬇️
app/routers/session_answers.py	86.66% <85.71%> (-6.89%)	⬇️
app/tests/test_session_answers.py	100.00% <100.00%> (ø)
app/tests/test_sessions.py	100.00% <100.00%> (ø)

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