chore: enable forcetypeassert linter, fix issues (#38804)

Enable [`forcetypeassert`](https://github.com/gostaticanalysis/forcetypeassert)
linter to prevent unchecked type assertions. ~650 issues fixed, most
fixes were clean, some use `setting.PanicInDevOrTesting`.

The only behaviour changes are where code would previously send a 500 error
or panic, a 4xx error is now emitted.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/math/block_renderer.go

@@ -49,7 +49,7 @@ func (r *BlockRenderer) writeLines(w util.BufWriter, source []byte, n gast.Node)
 }
 
 func (r *BlockRenderer) renderBlock(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
-	n := node.(*Block)
+	n := node.(*Block) //nolint:forcetypeassert // registered for KindBlock only
 	if entering {
 		codeHTML := giteaUtil.Iif[template.HTML](n.Inline, "", `<pre class="code-block is-loading">`) + `<code class="language-math">`
 		_, _ = w.WriteString(string(r.renderInternal.ProtectSafeAttrs(codeHTML)))