Paginate getEvents, extract event decoder, bigint transformer, contract expiry

[bobaivigitalpoint-ui]

Closes #1409
Closes #1410
Closes #1411
Closes #1412

• [Backend] Paginate Soroban getEvents instead of reading only the first page #1409 getEventsSinceLive now pages through getEvents using the response cursor (200 per page, bounded at 50 pages) and, when the RPC rejects startLedger as outside its retained range, logs it and re-seeds from the oldest available ledger
• [Backend] Move the event decoder out of the ~950-line soroban.service.ts #1410 moved GIST_EVENT_TOPICS, the event interfaces and decodeGistRegistryEvent into src/soroban/event-decoder.ts; soroban.service.ts re-exports them, so existing imports keep working. No behaviour change
• [Backend] IndexerState.lastProcessedLedger is typed number but the column is bigint #1411 added a bigint-to-number column transformer on IndexerState.lastProcessedLedger and dropped the manual Number(...) in IndexerService
• [Backend] API computes expires_at from the server clock instead of the contract's expiry #1412 GistsService.create now reads the expiry the contract assigned (getGist(...).expiresAt) and stores that, falling back to the server-clock estimate in mock mode or if the read fails; two tests added

Tests and build were not run.

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