Add migration to set DB constraint

[norkans7]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a database migration that introduces a check constraint to ensure only one segment field can be set at a time in the stats_pollstats table. The constraint validates that at most one of the four segment fields (age_segment_id, gender_segment_id, location_id, scheme_segment_id) contains a non-null value.

• Adds a PostgreSQL check constraint using num_nonnulls() function
• Includes a reverse migration to properly drop the constraint

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[Copilot]

Corrected spelling of 'CONTRAINT' to 'CONSTRAINT'.

[Copilot]

Corrected spelling of 'CONTRAINT' to 'CONSTRAINT'.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.94%. Comparing base (3ca2d9c) to head (4277edf).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           fewer-poll-stats-rows-2    #1305   +/-   ##
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  Coverage                    83.94%   83.94%           
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  Files                           48       48           
  Lines                         5899     5899           
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  Hits                          4952     4952           
  Misses                         947      947           

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