fix: broadcast context and status code improvements

[shruggr]

Summary

• Detach broadcast goroutines from request context using context.WithoutCancel so client disconnect doesn't cancel in-flight teranode submissions
• Use a shared broadcast context with 15s timeout per fan-out, cleaned up when all goroutines complete (not on early return)
• Set StatusCode on TransactionStatus at construction in submitToTeranodeSync: pass through teranode HTTP codes, 503 for network failures, 504 for timeouts
• Simplify collection loop from select-based to range over buffered channel

Test plan

• Verify broadcasts propagate to all endpoints even when caller returns early on first success
• Verify client disconnect mid-broadcast doesn't cancel remaining submissions
• Verify StatusCode is non-zero in SSE events and webhook callbacks
• Verify 503 for network failures, 504 for timeouts, passthrough for teranode 4xx/5xx
• Existing tests pass, linter clean

[mrz1836]

LGTM