fix: add http.Flusher to metrics responseWriter for SSE streaming

[umputun]

Metrics middleware wraps http.ResponseWriter to capture status codes, but the wrapper did not implement http.Flusher. This caused httputil.ReverseProxy to fall back to buffered mode — all SSE events were buffered until the backend response completed, then delivered in a single burst when MGMT_ENABLED=true.

Changes:

• Add Flush() method to responseWriter in app/mgmt/metrics.go that delegates to the underlying writer, following the same pattern as the existing Hijack() method
• Fix wrong error variable in app/main.go:303 — sizeParse error was wrapped with err (from a previous call) instead of perr
• Add tests for flush delegation, no-op on non-flushable writers

[Copilot]

Pull request overview

Fixes SSE streaming behavior when the management metrics middleware wraps http.ResponseWriter by ensuring the wrapper supports http.Flusher, and corrects an unrelated error-wrapping bug in run().

Changes:

• Add Flush() to the metrics responseWriter wrapper (delegating to the underlying writer when supported).
• Fix incorrect error variable used when wrapping sizeParse(opts.MaxSize) failures.
• Add unit tests covering flush delegation and safe behavior for non-flushable writers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
app/mgmt/metrics.go	Add Flush() to the responseWriter wrapper to preserve streaming behavior through middleware.
app/mgmt/server_test.go	Add tests validating Flush() delegation and no-op behavior when flush isn’t supported.
app/main.go	Fix error-wrapping to use perr from sizeParse instead of a stale err.

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