Count what the cache holds in the default cache_max_bytes (closes #184)

The default cache_max_bytes was 75% of the space free at startup. The
cache's own files are not free space, so a fuller cache got a smaller
limit after a restart and eviction then deleted most of it. The default
is now 75% of the sum of the free space and what the cache already holds
by its own size accounting, at least 500 MiB. The cache works it out
when it opens, after the database is open, so the computation and its
tests moved from internal/config to internal/imgcache. The config only
records whether cache_max_bytes was set; newCacheConfig in the handlers
turns the disk cache off only for an explicit 0, and is tested for an
omitted, a zero and a positive value.

Model: opus-5-5

internal/handlers/handlers.go

@@ -80,18 +80,25 @@ func (s *Handlers) WaitForProcessing(ctx context.Context) int {
 	return s.imgSvc.WaitForProcessing(ctx)
 }
 
+// newCacheConfig builds the image cache's configuration from cfg.
+// cache_max_bytes: 0 disables the disk cache entirely; any other value
+// is the eviction limit in bytes; when it is omitted, the cache works
+// out the default limit itself.
+func newCacheConfig(cfg *config.Config, log *slog.Logger) imgcache.CacheConfig {
+	return imgcache.CacheConfig{
+		StateDir:           cfg.StateDir,
+		CacheTTL:           imgcache.DefaultCacheTTL,
+		NegativeTTL:        imgcache.DefaultNegativeTTL,
+		MaxBytes:           cfg.CacheMaxBytes,
+		UseDefaultMaxBytes: !cfg.CacheMaxBytesExplicit,
+		DisableDiskCache:   cfg.CacheMaxBytesExplicit && cfg.CacheMaxBytes == 0,
+		Logger:             log,
+	}
+}
+
 // initImageService initializes the image cache and service.
 func (s *Handlers) initImageService() error {
-	// Create the cache. cache_max_bytes: 0 disables the disk cache
-	// entirely; any other value is the eviction limit in bytes.
-	cache, err := imgcache.NewCache(s.db.DB(), imgcache.CacheConfig{
-		StateDir:         s.config.StateDir,
-		CacheTTL:         imgcache.DefaultCacheTTL,
-		NegativeTTL:      imgcache.DefaultNegativeTTL,
-		MaxBytes:         s.config.CacheMaxBytes,
-		DisableDiskCache: s.config.CacheMaxBytes == 0,
-		Logger:           s.log,
-	})
+	cache, err := imgcache.NewCache(s.db.DB(), newCacheConfig(s.config, s.log))
 	if err != nil {
 		return err
 	}