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

The new package internal/providers builds the cache, the fetcher, the
processor, the image service, the session manager and the encrypted URL
generator from the config. providers.Option gives them to pixad's fx
graph, and handlers.New receives them built. The server tests build the
same graph, with only the fetcher's dial function replaced. 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, and
internal/providers works out the default cache_max_bytes. 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.

Model: opus-5-5

internal/imgcache/cache.go

@@ -32,23 +32,12 @@ type CacheConfig struct {
 	NegativeTTL time.Duration
 
 	// MaxBytes is the disk cache size limit in bytes that eviction
-	// enforces. Zero means no limit is enforced (no eviction).
+	// enforces. Zero turns the disk cache off entirely: no cache
+	// directories are created, lookups always miss, stores are no-ops,
+	// and the eviction goroutine only removes the expired negative cache
+	// rows.
 	MaxBytes int64
 
-	// UseDefaultMaxBytes makes NewCache replace MaxBytes with the
-	// default limit: 75% of the sum of the space free on the filesystem
-	// holding the cache and the bytes the cache already holds, at least
-	// DefaultCacheMaxBytesFloor. The config layer sets this when the
-	// operator omits cache_max_bytes.
-	UseDefaultMaxBytes bool
-
-	// DisableDiskCache turns the disk cache off entirely: no cache
-	// directories are created, lookups always miss, stores are
-	// no-ops, and the eviction goroutine only removes the expired
-	// negative cache rows. The config layer sets this when the
-	// operator configures cache_max_bytes: 0.
-	DisableDiskCache bool
-
 	// Logger receives accounting and eviction log output. A nil
 	// Logger means slog.Default().
 	Logger *slog.Logger
@@ -62,10 +51,6 @@ type Cache struct {
 	config     CacheConfig
 	log        *slog.Logger
 
-	// disabled means the disk cache is turned off entirely: lookups
-	// always miss, stores are no-ops, and no eviction runs.
-	disabled bool
-
 	// Eviction machinery. The channels are created in NewCache so
 	// stores can signal write pressure without racing StartEviction.
 	// evictionCancel, set by StartEviction, cancels the eviction
@@ -97,7 +82,7 @@ type Cache struct {
 	transforms         prometheus.Counter
 
 	// reconciliationPageSize is the most rows one read of a content table
-	// returns in the reconciliation pass. newCache sets it to
+	// returns in the reconciliation pass. NewCache sets it to
 	// defaultReconciliationPageSize; tests set it smaller.
 	reconciliationPageSize int
 
@@ -110,20 +95,9 @@ type Cache struct {
 
 // NewCache creates a new cache instance and registers its counters on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
-// /metrics serves.
+// /metrics serves. It makes no database call.
 func NewCache(
 	db *sql.DB, config CacheConfig, metricsRegistry prometheus.Registerer,
-) (*Cache, error) {
-	return newCache(db, config, metricsRegistry, defaultFreeSpaceProbe)
-}
-
-// newCache is NewCache with the free-space probe passed in, so tests
-// can fake the free space the default limit is worked out from.
-func newCache(
-	db *sql.DB,
-	config CacheConfig,
-	metricsRegistry prometheus.Registerer,
-	probe FreeSpaceProbeFunc,
 ) (*Cache, error) {
 	log := config.Logger
 	if log == nil {
@@ -134,7 +108,6 @@ func newCache(
 		db:               db,
 		config:           config,
 		log:              log,
-		disabled:         config.DisableDiskCache,
 		evictionPressure: make(chan struct{}, 1),
 		evictionDone:     make(chan struct{}),
 
@@ -150,7 +123,7 @@ func newCache(
 		return nil, err
 	}
 
-	if c.disabled {
+	if config.MaxBytes == 0 {
 		return c, nil
 	}
 
@@ -171,20 +144,6 @@ func newCache(
 	c.srcContent = srcContent
 	c.variants = variants
 
-	if config.UseDefaultMaxBytes {
-		limit, err := c.computeDefaultMaxBytes(context.Background(), probe)
-		if err != nil {
-			return nil, err
-		}
-
-		c.config.MaxBytes = limit
-
-		log.Info("computed default cache size limit from free space and cache contents",
-			"cache_max_bytes", limit,
-			"cache_dir", filepath.Join(config.StateDir, "cache"),
-		)
-	}
-
 	return c, nil
 }
 
@@ -196,7 +155,7 @@ func newCache(
 func (c *Cache) GetVariant(
 	cacheKey imagerequest.VariantKey,
 ) (io.ReadCloser, int64, string, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil, 0, "", ErrNotFound
 	}
 
@@ -214,7 +173,7 @@ func (c *Cache) StoreSource(
 	req *imagerequest.Request,
 	content io.Reader,
 ) (imagerequest.SourceKey, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return "", nil
 	}
 
@@ -259,7 +218,7 @@ func (c *Cache) StoreSource(
 func (c *Cache) StoreVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey, content io.Reader,
 ) error {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil
 	}
 
@@ -293,7 +252,7 @@ func (c *Cache) StoreVariant(
 func (c *Cache) LookupSource(
 	ctx context.Context, req *imagerequest.Request,
 ) (imagerequest.SourceKey, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return "", nil
 	}
 
@@ -345,7 +304,7 @@ func (c *Cache) StoreNegative(ctx context.Context, req *imagerequest.Request) er
 func (c *Cache) GetSourceContent(
 	key imagerequest.SourceKey,
 ) (io.ReadCloser, int64, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil, 0, ErrNotFound
 	}