Close the review gaps: CI lint, server timeouts, security headers

The initial scaffold left three holes the review caught. CI only checked
gofmt and go vet, so golangci-lint and the pre-commit hooks were advisory
rather than enforced. The HTTP server bounded only the header read, so a
slow or stalled peer could hold a connection indefinitely. And the browser
got no content-security policy at all, leaving the SPA's same-origin
assumption unenforced.

Add golangci-lint and pre-commit hook steps to the existing pre-commit
workflow, mirroring the estate's images so the required context name stays
ci/woodpecker/pr/pre-commit.
Bound the server with ReadTimeout, WriteTimeout, and IdleTimeout, keeping
the write budget generous enough for the poster proxy's streamed responses.
Stamp Content-Security-Policy, X-Content-Type-Options, and Referrer-Policy
onto every response from a single middleware wrapping the root handler.
Assert the headers across the API, UI, assets, probes, and rejections.
Guard the CSP's no-unsafe-inline assumption with a ui test that fails if a
shipped asset grows an inline script, style block, or event handler.
Extend the make pre-commit target to match the widened CI checks.

cmd/mediamark/main.go

@@ -42,6 +42,11 @@ func main() {
 		Addr:              cfg.Listen,
 		Handler:           server.New(cfg, lib, arrs, ui.Assets(), log).Handler(),
 		ReadHeaderTimeout: 10 * time.Second,
+		ReadTimeout:       30 * time.Second,
+		// The art proxy streams posters upstream-to-browser, so the write budget
+		// is the generous one.
+		WriteTimeout: 60 * time.Second,
+		IdleTimeout:  120 * time.Second,
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)