#522 Fix admin dashboard statistics returning zero counts

[Agaki00]

Description

Summary

• Query real counts for users, streams, active streams, and events from the last 24 hours.
• Inject PG_POOL into AdminStatsService with database error handling.
• Add integration coverage for active/inactive streams and event time filtering.
• Update the Swagger description for GET /admin/stats.
• Preserve the existing 60-second controller cache behavior.

Testing

• Workspace diagnostics pass.
• git diff --check passes.
• Typecheck and integration tests could not run because dependencies were unavailable in the environment.

Closes #522