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/imageprocessor/imageprocessor.go

@@ -11,7 +11,6 @@ import (
 	"time"
 
 	"github.com/davidbyttow/govips/v2/vips"
-	"sneak.berlin/go/pixa/internal/config"
 	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
@@ -79,10 +78,6 @@ type Result struct {
 // Images larger than this are rejected to prevent DoS via decompression bombs.
 const MaxInputDimension = 8192
 
-// DefaultMaxInputBytes is the default maximum input size in bytes (50 MiB).
-// This matches the default upstream fetcher limit.
-const DefaultMaxInputBytes = 50 << 20
-
 // ErrInputTooLarge is returned when input image dimensions exceed MaxInputDimension.
 var ErrInputTooLarge = errors.New("input image dimensions exceed maximum")
 
@@ -142,35 +137,22 @@ type ImageProcessor struct {
 	processingWaitTimeout time.Duration
 }
 
-// Params holds configuration for creating an ImageProcessor.
-// Zero values use sensible defaults (MaxInputBytes defaults to DefaultMaxInputBytes).
+// Params holds configuration for creating an ImageProcessor. Both fields
+// must be at least 1.
 type Params struct {
 	// MaxInputBytes is the maximum allowed input size in bytes.
-	// If <= 0, DefaultMaxInputBytes is used.
 	MaxInputBytes int64
 	// MaxConcurrentProcessing is the most images processed at once.
-	// If <= 0, config.DefaultMaxConcurrentProcessing() is used.
 	MaxConcurrentProcessing int
 }
 
 // New creates a new image processor with the given parameters.
-// A zero-value Params{} uses sensible defaults.
 func New(params Params) *ImageProcessor {
 	initVips()
 
-	maxInputBytes := params.MaxInputBytes
-	if maxInputBytes <= 0 {
-		maxInputBytes = DefaultMaxInputBytes
-	}
-
-	maxConcurrentProcessing := params.MaxConcurrentProcessing
-	if maxConcurrentProcessing <= 0 {
-		maxConcurrentProcessing = config.DefaultMaxConcurrentProcessing()
-	}
-
 	return &ImageProcessor{
-		maxInputBytes:         maxInputBytes,
-		processingSemaphore:   make(chan struct{}, maxConcurrentProcessing),
+		maxInputBytes:         params.MaxInputBytes,
+		processingSemaphore:   make(chan struct{}, params.MaxConcurrentProcessing),
 		processingWaitTimeout: ProcessingWaitTimeout,
 	}
 }