Limit each client's requests to the image routes (closes #236)

The new setting image_requests_per_minute (PIXA_IMAGE_REQUESTS_PER_MINUTE),
default 600, caps the GET and HEAD requests one client sends to /v1/image/
and /v1/e/ per minute; the next is answered 429 with Retry-After and the
image routes' JSON error body. It uses the login limit's RateLimit
middleware, so clients are told apart the same way, behind the CORS
middleware, so a preflight is not counted. 0 turns it off, as
script/loadtest does; a negative or non-integer value stops startup.

TestEnvironmentSetsEveryKey gains the new variable, under the standing
approval in #64. To stay within the
linters' limits, the sentry_dsn check moves unchanged into
validateSentryDSN, and the cache_max_bytes explicit flag is set in one
line.

Model: opus-5-5

internal/handlers/image.go

@@ -206,6 +206,16 @@ func parseExpires(query url.Values) (time.Time, error) {
 	return time.Unix(exp, 0), nil
 }
 
+// HandleTooManyRequests answers 429 Too Many Requests with the JSON error
+// body the image routes send for their other errors. The image routes' rate
+// limit calls it for a request over the limit.
+func (s *Handlers) HandleTooManyRequests() http.HandlerFunc {
+	return func(w http.ResponseWriter, _ *http.Request) {
+		s.respondError(w, "too many requests, try again later",
+			http.StatusTooManyRequests)
+	}
+}
+
 // respondImageError maps image retrieval errors to HTTP responses.
 func (s *Handlers) respondImageError(
 	w http.ResponseWriter, req *imgcache.ImageRequest, err error,