perf(ab-testing): add indexes to experiment entity + migration - #1371

Overview

This PR adds database indexes to the experiment entity so common lookup, filter, sort, and foreign-key query paths no longer trigger sequential scans as the table grows. Indexes are declared via @Index on the entity and created by a reviewable migration so existing databases are updated without relying on synchronize.

Related Issue

Changes

🗂️ Experiment Entity Indexing

• [MODIFY] src/ab-testing/entities/experiment.entity.ts

  • Add @Index decorators covering the columns used in WHERE / ORDER BY / JOIN / foreign-key lookups.
  • Use single-column indexes for standalone lookups and composite indexes where columns are queried together (e.g. status + createdAt), avoiding redundant/overlapping indexes.

• [ADD] src/migrations/1756475492000-add-experiment-indexes.ts

  • up() creates the same indexes declared on the entity so existing databases are brought in line.
  • down() drops them, keeping the migration reversible.
  • Index names are explicit and deterministic to match the entity declarations and prevent duplicates.

Verification Results

# Compile / typecheck
npm run build

# Run migration against a database with existing data
npm run typeorm migration:run
npm run typeorm migration:revert   # confirm down() is clean

# Tests
npm test -- experiment

Note: fill in the actual output above from a real run. I have not executed these; results should reflect your local/CI run, not placeholder numbers.

Closes #1224