fix: close sse stream on abort

[Divv1524]

Summary

Fixed the SSE stream lifecycle in /api/stream so server-side resources are cleaned up reliably when clients disconnect, streams are canceled, or connections become stale.

Closes #1608

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Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

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Changes Made

• Centralized SSE cleanup into one idempotent close path
• Safely closes the ReadableStream controller on client abort
• Added guarded controller.enqueue() handling to prevent writes to closed streams
• Added a 5-minute maximum SSE connection duration
• Added cleanup support for canceled stream readers and already-aborted requests
• Added regression tests for SSE cleanup behavior

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How to Test

Steps for the reviewer to verify this works:

1. Run npm run lint
2. Run npm run type-check
3. Run npm run build
4. Run vitest run test/sse-stream-route.test.ts
5. Confirm SSE stream connections release their active connection slot after abort, cancel, or timeout

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Screenshots (if UI change)

N/A

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Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• Proper keyboard navigation tested
• Responsive UI verified
• Accessibility labels added where needed

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Additional Notes

Full npm test was also run. The focused SSE test suite passed, but the full suite currently has unrelated pre-existing failures outside this change.

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