Bound concurrent image processing and upstream fetches (closes #64)

Nothing bounded total in-flight work, so a burst of cache misses across
hosts could exhaust memory. Two settings now do: max_concurrent_processing
(default the CPUs Go uses) and upstream_connections (default 64, beside
the per-host limit). A request that finds either full waits up to 10
seconds, then gets 503; a slot is released on every path. No request
holds source bytes while it waits: a cached source is read only after
the processing slot is taken, and a fetched one only while it holds its
upstream connection. libvips runs one worker thread per image with its
operation cache off. Both waits count toward downstream_timeout, as the
README says.

Model: opus-5-5

internal/handlers/server_busy_internal_test.go

@@ -0,0 +1,49 @@
+package handlers
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestServerBusyAnswers503 checks that both image routes answer 503 with
+// a clear error when the image service gives up waiting for a free upstream
+// connection or processing slot, wrapped as the service wraps them.
+func TestServerBusyAnswers503(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.jpg"}
+
+	for _, err := range []error{
+		fmt.Errorf("upstream fetch failed: %w", httpfetcher.ErrTooManyConnections),
+		fmt.Errorf("image processing failed: %w", imageprocessor.ErrTooManyImages),
+	} {
+		plain := httptest.NewRecorder()
+		h.respondImageError(plain, req, err)
+
+		encrypted := httptest.NewRecorder()
+		h.handleImageError(encrypted, err)
+
+		for route, rec := range map[string]*httptest.ResponseRecorder{
+			"/v1/image/": plain, "/v1/e/": encrypted,
+		} {
+			if rec.Code != http.StatusServiceUnavailable {
+				t.Errorf("%s for %v: status = %d, want %d",
+					route, err, rec.Code, http.StatusServiceUnavailable)
+			}
+
+			if !strings.Contains(rec.Body.String(), "server busy, try again later") {
+				t.Errorf("%s for %v: body = %q, want the server busy error",
+					route, err, rec.Body.String())
+			}
+		}
+	}
+}