feat: DB-tracked cache size accounting with background LRU eviction

Migration 002 adds a variant_content table (processed variants were
untracked on disk) and an LRU timestamp on source_content. Total usage
is two SUMs, never a directory scan on the hot path; hits touch LRU
timestamps best-effort. A background goroutine evicts globally
least-recently-used entries (variants and source blobs merged) until
usage is under MaxBytes, woken by a periodic ticker and by non-blocking
write-pressure notifications from stores. Evicting a source blob
deletes all source_metadata rows referencing it plus its
source_content row in one transaction before the file is unlinked, so
multi-referenced blobs are removed only with all their references and
rows never point at deleted files; JSON sidecars are cleaned up too. A
one-time startup reconciliation walk adopts untracked variant files,
drops rows whose files are missing, removes unreachable source blobs,
and sweeps stale temp files. CacheConfig.DisableDiskCache turns the
disk cache off entirely (config maps cache_max_bytes: 0 to it): no
directories, lookups miss, stores no-op, no evictor. Handlers wire the
limit, start eviction on startup, and stop it on shutdown.

internal/handlers/handlers.go

@@ -53,6 +53,13 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
+		OnStop: func(_ context.Context) error {
+			if s.imgCache != nil {
+				s.imgCache.StopEviction()
+			}
+
+			return nil
+		},
 	})
 
 	return s, nil
@@ -60,11 +67,15 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 
 // initImageService initializes the image cache and service.
 func (s *Handlers) initImageService() error {
-	// Create the cache
+	// 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,
+		StateDir:         s.config.StateDir,
+		CacheTTL:         imgcache.DefaultCacheTTL,
+		NegativeTTL:      imgcache.DefaultNegativeTTL,
+		MaxBytes:         s.config.CacheMaxBytes,
+		DisableDiskCache: s.config.CacheMaxBytes == 0,
+		Logger:           s.log,
 	})
 	if err != nil {
 		return err
@@ -72,6 +83,10 @@ func (s *Handlers) initImageService() error {
 
 	s.imgCache = cache
 
+	// Background eviction: startup reconciliation, then periodic and
+	// write-pressure passes. No-op when the disk cache is disabled.
+	cache.StartEviction(imgcache.DefaultEvictionInterval)
+
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()
 	fetcherCfg.AllowHTTP = s.config.AllowHTTP