feat: add index for member api (CM-764)

[ulemons]

Add database indexes to improve member query performance

Summary

This PR introduces three new indexes aimed at optimizing search and ranking queries
executed by the /member/query endpoint and related aggregation logic.

Rationale

• idx_msa_segment_activitycount_desc_member
  Enables efficient retrieval of the top active members within a segment,
  removing the need for expensive sorts on large result sets.

• idx_members_displayname_trgm
  Improves performance of text searches by display name
  (LOWER(displayName) LIKE '%term%'), commonly used in the search feature.

• idx_memberidentities_email_verified_trgm
  Accelerates lookups of verified email identities
  (LOWER(value) LIKE '%term%'), reducing scan cost on memberIdentities.

---

Note

Add three concurrent Postgres indexes to speed member queries, display name search, and verified email lookups.

• Database Migration (backend/src/database/migrations/V1762789440__optimize_member_api.sql):
  • Indexes:
    • idx_msa_segment_activitycount_desc_member on public."memberSegmentsAgg" ("segmentId", "activityCount" DESC, "memberId").
    • idx_members_displayname_trgm GIN on public."members" using LOWER("displayName") gin_trgm_ops.
    • idx_memberidentities_email_verified_trgm GIN on public."memberIdentities" using LOWER("value") gin_trgm_ops with partial filter WHERE verified = true AND type = 'email'.

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