Share one server-sent events helper across the four streams #325

Four routes hand-roll the same event stream code (encoder and send, an opening comment, a 25 second heartbeat, cleanup on abort, and the same four headers), so a fix to one stream never reaches the others.

Where to start

1. Add one small helper (for example backend/src/util/sse.ts) that owns the ReadableStream, send and sendEvent, the heartbeat as a named constant, abort cleanup and the response headers.
2. Move the run stream, the thread stream, the chat stream and the changes stream onto it, keeping every frame they send byte for byte.
3. Add a unit test for the helper: headers, the opening comment, heartbeat and cleanup on abort.

Code to look at:

• backend/src/runs/routes.ts:589
• backend/src/runs/routes.ts:676
• backend/src/runs/routes.ts:853
• backend/src/runs/routes.ts:1004
• backend/src/chat/routes.ts:149
• backend/src/chat/routes.ts:221
• backend/src/runs/changes-route.ts:33
• backend/src/runs/changes-route.ts:58

Done when

• The literal 25_000 heartbeat and the four-header block each appear once in backend/src.
• Existing tests for these streams pass unchanged (backend/test/thread-events.test.ts, backend/test/chat-routes.test.ts and the run stream tests).

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