feat: expose pg-pool metrics, add indexer-state race guard, add admin rate limiter

[Smartdevs17]

Summary

This PR addresses three assigned issues:

Closes #1023

[Backend] pg-pool.ts does not expose pool metrics (total/idle/waiting) to the admin metrics endpoint

• Added getPoolMetrics() function in pg-pool.ts to expose totalCount, idleCount, and waitingCount
• Exported pool instance from prisma.ts
• Added pgPool section to admin metrics response

Closes #1022

[Backend] indexer-state.ts singleton row has no migration guard against concurrent first-insert race

• Added ensureIndexerState() function with catch-and-retry pattern for P2002 unique-constraint violations
• Updated soroban-event-worker.ts to use the new safe initializer
• Added tests simulating duplicate insert scenarios

Closes #1020

[Backend] admin.routes.ts registers admin endpoints with no route-level rate limiting

• Created admin-rate-limiter.middleware.ts with 30 requests/minute limit
• Applied rate limiter to admin router after auth middleware
• Added test for the rate limiter

Changes

Modified files:

• backend/src/lib/pg-pool.ts - Added getPoolMetrics() function
• backend/src/lib/prisma.ts - Exported pool instance and getPoolMetrics
• backend/src/lib/indexer-state.ts - Added ensureIndexerState() with race condition guard
• backend/src/routes/v1/admin.routes.ts - Added rate limiter and pool metrics
• backend/src/workers/soroban-event-worker.ts - Updated to use ensureIndexerState()
• backend/tests/pg-pool.test.ts - Added test for getPoolMetrics()

New files:

• backend/src/middleware/admin-rate-limiter.middleware.ts - Admin rate limiter middleware
• backend/tests/admin-rate-limiter.test.ts - Rate limiter tests
• backend/tests/indexer-state.test.ts - Indexer state race condition tests