fix(backend): use BigInt for on-chain streamId (u64-safe)

[TCROWN10]

Summary

• Change Stream.streamId and StreamEvent.streamId from Prisma Int (Postgres int4) to BigInt, with a migration that widens both columns and recreates the FK.
• Stop coercing decoded Soroban u64 IDs with Number(...); parse route/body IDs via parseStreamId() and serialize BigInt safely for JSON/SSE.
• Add tests for u64 IDs above int4 max (3_000_000_000n) through the worker and validator.

Closes #829

Test plan

• Focused vitest suite for stream-id helpers, worker large-id path, and stream action/integration expectations
• CI green (Postgres migration applied in CI)
• Confirm a stream with streamId > 2147483647 persists and is readable via GET /v1/streams/:id