Return and pass on request IDs, and give /v1/e/ ETag, 304 and HEAD (closes #84)

Every response carries X-Request-Id, the upstream fetch sends it, and
the "upstream fetched", "image converted" and "image served" lines log
it as request_id. pixa's own RequestID middleware keeps a request's own
ID only when it is at most 64 letters, digits, '-', '_' or '.', and
otherwise makes a random one with crypto/rand, so nothing a client
chooses freely and nothing about the host reaches upstream. /v1/e/ now
sets ETag, answers a matching If-None-Match with 304 and is routed for
HEAD, through notModified, which both image handlers call. No Vary is
added: go-chi/cors already sends Vary: Origin.

Model: opus-5-5

internal/server/routes_internal_test.go

@@ -0,0 +1,27 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestEncryptedImageRouteAnswersHEAD verifies that HEAD on the encrypted image
+// route reaches its handler, as GET does, instead of being answered 405 Method
+// Not Allowed. The handler refuses a token it cannot decrypt with 400, so that
+// status shows the request got through.
+func TestEncryptedImageRouteAnswersHEAD(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodHead, encryptedImagePath, nil))
+	t.Logf("status %d", rec.Code)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d from the encrypted image handler",
+			rec.Code, http.StatusBadRequest)
+	}
+}