Build the image pipeline in pixad's fx graph (closes #313)

internal/app now provides the cache, the fetcher, the processor, the
image service, the session manager and the encrypted URL generator, and
handlers.New receives them built. database.New opens the database, so
these parts can read it while fx builds them; the cache's stop hook
still runs before the database's.

Each setting's default is declared in internal/config, the default
cache_max_bytes included, which internal/app works out before it builds
the cache. The cache's MaxBytes is the one value for the disk cache
limit, and 0 turns the disk cache off. The fetcher and the processor
take explicit values. The server tests build the pipeline themselves,
as they cannot import internal/app.

Model: opus-5-5

internal/imgcache/testutil_internal_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/database"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 )
 
 // Shared test data literals, extracted as constants for goconst.
@@ -185,16 +186,22 @@ func SetupTestService(
 	cache, err := NewCache(db, CacheConfig{
 		StateDir:    tmpDir,
 		NegativeTTL: 5 * time.Minute,
+		MaxBytes:    1 << 30,
 	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("failed to create cache: %v", err)
 	}
 
 	svc, err := NewService(&ServiceConfig{
-		Cache:      cache,
-		Fetcher:    httpfetcher.NewMock(mockFS),
-		SigningKey: cfg.signingKey,
-		Allowlist:  cfg.allowlist,
+		Cache:   cache,
+		Fetcher: httpfetcher.NewMock(mockFS),
+		Processor: imageprocessor.New(imageprocessor.Params{
+			MaxInputBytes:           50 << 20,
+			MaxConcurrentProcessing: 2,
+		}),
+		SigningKey:      cfg.signingKey,
+		Allowlist:       cfg.allowlist,
+		MaxResponseSize: 50 << 20,
 	})
 	if err != nil {
 		t.Fatalf("failed to create service: %v", err)