fix: consolidate solana-relay Knex connection pools

[dylanjeffers]

Problem

The solana-relay service had 8 route/middleware files each calling initializeDiscoveryDb() at module scope, creating 8 independent Knex connection pools. Each pool defaults to max 10 connections, so up to 80 connections could be open simultaneously, exhausting Postgres under load.

This caused: Knex: Timeout acquiring a connection. The pool is probably full.

Fix

Extracted a single shared Knex singleton at src/db.ts and updated all 8 callers to import from it.

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Latest commit: e7a96ea

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