Test the image proxy flow end to end (closes #80)

TestImageProxyFlow in internal/server starts the database, handlers and
middleware from the constructors pixad uses, with a fresh state
directory, and replaces only the upstream origin with an httptest
server. For a resize with a format change and for orig it checks a 200
MISS with the right type and size, then a HIT after one upstream request,
and the files and rows the cache keeps. Two optional test seams make
that possible: httpfetcher.Config.DialContext and handlers.Params.Fetcher.
pixad sets neither and the config file and environment cannot, and tests
show production still uses the checked dialer and builds its own fetcher.

Model: opus-5-5

internal/imgcache/service.go

@@ -42,7 +42,8 @@ type Service struct {
 type ServiceConfig struct {
 	// Cache is the cache instance
 	Cache *Cache
-	// FetcherConfig configures the upstream fetcher (ignored if Fetcher is set)
+	// FetcherConfig configures the upstream fetcher built when Fetcher is
+	// not set. Its AllowHTTP and MaxResponseSize are used either way.
 	FetcherConfig *httpfetcher.Config
 	// Fetcher is an optional custom fetcher (for testing)
 	Fetcher httpfetcher.Fetcher