Fix/composite indexes 258

[kimanicode]

Summary

Adds dashboard-focused composite indexes so filtered campaign and aid package queries can use ordered composite access paths.

• Fixes Claim @@index([campaignId, status]) indentation
• Adds Campaign @@index([orgId, status, createdAt])
• Adds AidPackage @@index([campaignId, status, createdAt])
• Adds a Prisma migration for the new indexes
• Documents the intended dashboard query shapes in docs/db/indexes.md

Testing

DATABASE_URL="file:./dev.db" npx prisma validate --schema prisma/schema.prisma
npm run build
EXPLAIN ANALYZE was not run locally because this repo is currently configured for SQLite, while that acceptance check requires PostgreSQL.

### Screenshots
N/A

### Checklist
- [x] Tests added or updated for new/changed behavior
- [x] Existing tests pass
- [x] No secrets, keys, or seed phrases committed
- [x] Follows the coding conventions in the relevant service README
- [x] PR is focused on a single concern

[kilodesodiq-arch]

LGTM