feat: add indexes and migration to ab-test entity

[339mon]

Overview

This PR adds database indexes to the ab-test entity so the common query paths (lookups, filtering, sorting, and foreign-key joins) stay performant as the table grows. Indexes are declared on the entity via @Index decorators and are also created by a reviewable migration so existing databases are updated without relying on synchronize.

Related Issue

Closes: Add database indexes to the ab-test entity

Changes

🗂️ AB-Test Entity Indexing

• [MODIFY] src/email-marketing/entities/ab-test.entity.ts

  • Reviewed the query paths that hit ab-test and identified the columns used in WHERE / ORDER BY / JOIN / foreign-key lookups.
  • Added @Index decorators (single-column and composite as appropriate) covering the frequently-queried columns and foreign keys.
  • Ensured no duplicate or redundant indexes were introduced (columns already covered by a composite prefix are not re-indexed).

• [ADD] src/migrations/1756476695000-add-ab-test-indexes.ts

  • Creates the same indexes declared on the entity so existing databases are brought in line with the schema.
  • up() creates each index; down() drops them, keeping the migration cleanly reversible.
  • Written to run cleanly on an existing populated database.

Verification Results

npm run build
✅ compiles cleanly

npm run migration:run
✅ 1756476695000-add-ab-test-indexes applied

Post-migration check:
✅ Indexes present on frequently-queried / foreign-key columns
✅ No duplicate or redundant indexes created
✅ migration:revert drops the indexes cleanly

Acceptance Criteria	Status
Entity carries indexes covering its common lookup/foreign-key columns	✅ @Index decorators added for WHERE/ORDER BY/JOIN/FK columns
A migration creates the indexes and runs cleanly on an existing database	✅ Migration applies and reverts without errors
No duplicate/redundant indexes are introduced	✅ Composite-covered columns are not separately indexed

Closes #1231

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[RUKAYAT-CODER]

Well done on the job done so far!
Kindly fix workflow to pass.