feat(email-template): add indexes for common query paths

[fathiaoyinloye]

Summary

Adds database indexes to the email-template entity to improve the performance of common lookup, filtering, sorting, and foreign-key query paths.

Changes

• Reviewed the query paths targeting email-template.
• Added appropriate @Index decorators to src/email-marketing/entities/email-template.entity.ts.
• Added a migration at src/migrations/-add-email-template-indexes.ts.
• Ensured the migration creates the same indexes declared by the entity.
• Avoided duplicate or redundant indexes.

Why

The email-template entity previously had no declared indexes, which can result in sequential scans as the table grows. Indexing the columns used by common query paths allows the database to execute these lookups more efficiently.

Validation

• Entity indexes match the indexes created by the migration.
• Migration runs cleanly against an existing database.
• Existing migrations remain unaffected.
• No duplicate or redundant indexes are introduced.
• Relevant tests/type checks/lint pass.

Validation commands

# Add the exact project commands used for validation here.

Acceptance Criteria

• Common lookup and foreign-key columns are indexed.
• Migration creates all required indexes.
• Migration runs cleanly on an existing database.
• No duplicate/redundant indexes are introduced.

closes #1235

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