[backend] Add index for expectations

[savacano28]

Proposed changes

• Add index on expectations

Related issues

• Closes #ISSUE-NUMBER

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.15%. Comparing base (5874aaf) to head (cce83f8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1669      +/-   ##
============================================
+ Coverage     33.11%   33.15%   +0.04%     
- Complexity     1584     1586       +2     
============================================
  Files           550      551       +1     
  Lines         14421    14431      +10     
  Branches        873      873              
============================================
+ Hits           4775     4785      +10     
  Misses         9425     9425              
  Partials        221      221              

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