Activity Feed Database Index

Database migration that defines the PostgreSQL indexes required for performant cursor-based pagination and filter queries on the activity feed. Ensures that ordering by `created_at` combined with organization/local-association scoping does not result in full table scans as the activities table grows.

Feature: Activity Feed

activity-feed-index

• Define composite index on (organization_id, created_at DESC) for feed ordering
• Define index on (local_association_id, created_at DESC) for coordinator-scoped queries
• Define index on (status) for status filter performance
• Provide migration script for deployment