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.

Makefile

@@ -17,6 +17,8 @@ all: build
 pre-commit:
 	test -z "$$(gofmt -l .)"
 	go vet ./...
+	golangci-lint run ./...
+	uvx pre-commit run --all-files
 
 build:
 	@for b in $(BINARIES); do \