feat(backend): cache listServices to reduce RPC simulation calls

[leocagli]

Closes #6

Summary

Every call to GET /api/services triggered a Soroban simulation call to list_services_page — one per page, per request. With concurrent users or the frontend's 30s auto-refresh, this creates unnecessary load on the RPC node.

• Added a per-key in-memory cache in contract.js keyed on category:page:pageSize
• TTL = 30s (aligned with the frontend SWR refreshInterval)
• invalidateServicesCache() is called automatically inside registerServiceOnChain() so newly-registered services appear immediately without waiting for TTL expiry
• Cache is module-level (process-scoped) — survives across requests, resets on process restart

Test plan

• First request populates cache; second identical request within 30s returns cached data (no RPC call)
• After 30s the cache entry expires and the next request fetches fresh data
• After registerServiceOnChain(), the next GET /api/services shows the new service (cache was invalidated)
• Different category/page/pageSize combinations each get their own cache entry

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Reviewing files that changed from the base of the PR and between 6fb6da9 and f0124c3.

📒 Files selected for processing (1)

• backend/src/lib/contract.js

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