[feature] Beef up our AI opt-outs (#3165)

* [chore] Synchronise our robots.txt with upstream

* [feature] Add headers to escape AI crawlers

This adds 2 headers that a number of AI crawlers respect to signal that
content should not be included in their datasets.

internal/middleware/extraheaders.go

@@ -44,5 +44,12 @@ func ExtraHeaders() gin.HandlerFunc {
 		//
 		// See: https://github.com/patcg-individual-drafts/topics
 		c.Header("Permissions-Policy", "browsing-topics=()")
+
+		// Some AI scrapers respect the following tags to opt-out
+		// of their crawling and datasets.
+		c.Header("X-Robots-Tag", "noimageai")
+		// c.Header calls .Set(), but we want to emit the header
+		// twice, not override it.
+		c.Writer.Header().Add("X-Robots-Tag", "noai")
 	}
 }