fix: throttle transactions feed

[NateIsern]

Motivation

• The plural /api/transactions feed performs expensive daemon RPC fan-out (default getrawmempool + up to 20 getmempoolentry calls) but was only covered by the permissive global limiter, allowing unauthenticated clients to generate large backend load; this change bounds per-IP request cost.

Description

• Apply the existing strictLimiter to the /api/transactions route in server/index.ts so the feed is protected the same way as other expensive RPC-backed endpoints.

Testing

• Ran npm test which completed successfully (all test files passed).
• Ran npm run typecheck which succeeded.
• Attempted npm test -- --runInBand which failed because Vitest does not support the --runInBand flag, and git diff --check returned clean results.

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Codex Task

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