fix(db): prevent duplicate member segment affiliations

[prembharne]

Fixes #2840

Description:
This PR introduces a strict deduplication migration for the memberSegmentAffiliations table and enforces a UNIQUE index that handles NULL date values natively using COALESCE.

Additionally, we added a robust pre-insert SELECT check in memberSegmentAffiliationRepository.ts's createOrUpdate function. This prevents unhandled 500 Constraint Errors and updates seamlessly when exact matches are triggered, ensuring consistency across segments.

Changes:

• Added .sql cleanup and indexing migration.
• Graceful TypeScript fail-safes in backend integration preventing blind inserts.

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Note

Medium Risk
Adds a data-cleanup migration and a new unique index on an existing table, which can impact large datasets and fail if assumptions about NULL/default coercion don’t hold. Runtime changes are small but affect write-path behavior in createOrUpdate.

Overview
Introduces a migration that removes exact duplicate rows in memberSegmentAffiliations (keeping the lowest id) and then adds a unique index that treats NULL organizationId/dateStart/dateEnd as equal via COALESCE to prevent future duplicates.

Updates MemberSegmentAffiliationRepository.createOrUpdate to first SELECT an exact-match affiliation using IS NOT DISTINCT FROM (NULL-safe equality) and, if found, reuse it (while still calling updateAffiliation) instead of blindly inserting and risking a uniqueness/constraint error.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Timestamp type mismatch will cause migration to fail

High Severity

The dateStart and dateEnd columns are TIMESTAMP WITH TIME ZONE (per migration V1691658076), but the COALESCE sentinel values use ::timestamp (without time zone). Mixing timestamptz and timestamp in COALESCE triggers an implicit cast that PostgreSQL considers non-immutable (it depends on session timezone). Since expression indexes require all functions to be IMMUTABLE, the CREATE UNIQUE INDEX statement will fail at runtime, blocking the migration entirely. The cast needs to be ::timestamptz instead.