chore(#620): drop unused stellar-sdk dependency

[Fury03]

Closes #620

Summary

• Removes stellar-sdk from backend/package.json — no file under backend/src imports from stellar-sdk; all imports use @stellar/stellar-sdk
• Regenerates package-lock.json to reflect the removal

Test plan

• npm run build passes in backend/
• npm test passes in backend/
• Confirm grep -rn "from 'stellar-sdk'" backend/src returns nothing

[ogazboiz]

your branch is green, but it went conflicting after the ci fixes landed on main: #969 and #974 touched backend/Dockerfile and backend/src/workers/soroban-event-worker.ts, which this pr also edits. please rebase on main and it should be good to go: git fetch origin && git rebase origin/main && git push --force-with-lease. if any of your Dockerfile or soroban-event-worker changes overlap what #969 already did (copying prisma/ into the runner, and the import type { Prisma } from '../generated/prisma' fix), drop those now-redundant bits and keep just your actual change. i'll re-review and merge once it's rebased and ci is green.

[Fury03]

@ogazboiz You can review now

[ogazboiz]

safe removal, the backend imports @stellar/stellar-sdk (v15, a different package) not the bare stellar-sdk@13 you're dropping, and grep confirms zero imports of the bare one. dead dependency. merging.