feat(mw): add content-type and response middleware

Add Content-Type enforcement middleware and generic JSON response helpers to standardize API responses.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/mw/contenttype.go

@@ -0,0 +1,13 @@
+package mw
+
+import "net/http"
+
+// JsonContentType sets Content-Type: application/json on all API responses.
+// Individual handlers that need to override (e.g. binary/image responses)
+// should set their own Content-Type header after this middleware runs.
+func JsonContentType(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		next.ServeHTTP(w, r)
+	})
+}