Harden SSE notification loop

[upsetbit]

Closes #95.

Summary

• buffer the SSE notification event channel to avoid blocking the producer on every consumer tick
• preserve and log WaitForNotification errors when they are not normal context cancellation
• guard nil notifications before reading the payload

Local validation

• go test ./internal/server/handlers ./internal/server
• devbox run -- just lint
• just test-race

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