Give every JSON answer @meta, errors included (closes #201)

A panic in a JSON route's handler answered 500 with an empty body,
because the router's recoverer sits outside the middleware that adds
`@meta`. `JSONValidationMiddleware` now wraps the handler in the same
recoverer, and answers the panic with the JSON error body.

`/status.json` and the health check answered chi's empty 405 to any
method they do not take. They now answer with the same JSON 405 as the
routes under `/api/v1` and `/ip`.

The home page's IP lookup card showed only the answer's `data`. It now
shows the whole answer, `@meta` included.

A table test checks `@meta` on every JSON route, in its successes and
its errors.

Model: opus-5-5

internal/server/middleware.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
 )
 
 // responseWriter wraps http.ResponseWriter to capture the response
@@ -154,7 +155,8 @@ func deadlinePassed(ctx context.Context) bool {
 // has passed, it answers 504 with the error message "Request timeout". A 4xx
 // keeps its status, with the status's standard text as the message. For other
 // errors, it returns "Internal server error", with status 500 if the original
-// status was 200.
+// status was 200. A panic in the handler answers 500 with "Internal server
+// error" too.
 //
 // Non-JSON responses are passed through unchanged.
 func JSONValidationMiddleware(next http.Handler) http.Handler {
@@ -166,8 +168,9 @@ func JSONValidationMiddleware(next http.Handler) http.Handler {
 			statusCode:     http.StatusOK,
 		}
 
-		// Serve the request
-		next.ServeHTTP(rw, r)
+		// Serve the request. Recoverer logs a panic in the handler and turns it
+		// into a 500 without a body, which gets the JSON error body below.
+		middleware.Recoverer(next).ServeHTTP(rw, r)
 
 		// Check if it's meant to be a JSON response
 		contentType := rw.Header().Get("Content-Type")