feat: implement SorobanEventWorker with idempotency logic and unit tests

[sweetesty]

Closes #543

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This PR fixes handleStreamPaused and handleStreamResumed to use the same find-then-upsert-with-skip dedup guard as every other event handler in the worker.

Root cause
All other handlers (handleStreamCreated, handleTokensWithdrawn, handleFeeCollected, etc.) check tx.streamEvent.findUnique({ where: { transactionHash_eventType } }) and skip or upsert to avoid duplicates on replay. handleStreamPaused and handleStreamResumed called tx.streamEvent.create() directly with no guard, causing a unique-constraint violation (@@unique([transactionHash, eventType]) in schema.prisma) when /v1/admin/indexer/replay replayed a ledger containing pause or resume events.

Fix — backend/src/workers/soroban-event-worker.ts

• handleStreamPaused updated to use findUnique({ where: { transactionHash_eventType } }) before writing; skips if a matching row already exists, upserts if not
• handleStreamResumed updated with the same pattern
• Both handlers now behave identically to the rest of the worker under replay — restart-safe and consistent

Test

• Worker test extended to replay a ledger containing a pause + resume event twice
• Asserts no duplicate rows exist after the second replay
• Asserts no error is thrown on the second replay
• Confirms the existing dedup behaviour of other handlers is unaffected

Acceptance criteria met:

• ✅ handleStreamPaused uses find-then-upsert-with-skip pattern
• ✅ handleStreamResumed uses find-then-upsert-with-skip pattern
• ✅ Replay of a pause+resume ledger produces no duplicate rows and no thrown error

[ogazboiz]

hey, #706 just merged and fixes the same test-load issue your branch is failing on (5 test files showing '0 tests' due to broken auth mocks). please rebase:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

CI should clear once you pick up the auth mock fix.