Remove expired negative cache rows in the eviction loop (closes #244)

A negative cache row was deleted only when its source URL was requested
again after it expired, and CleanExpired was never called, so a client
asking for a new missing path each time grew the database without
limit. The eviction goroutine now calls CleanExpired at startup and on
every periodic tick, so a row stays at most NegativeTTL plus one
interval. The goroutine also starts with the disk cache disabled, where
reconciliation and eviction already do nothing, since the negative
cache is used either way.

Model: opus-5-5

internal/handlers/handlers.go

@@ -124,7 +124,8 @@ 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.
+	// write-pressure passes, and the removal of expired negative cache
+	// rows, which is all it does when the disk cache is disabled.
 	cache.StartEviction(imgcache.DefaultEvictionInterval)
 
 	// Writes the counts requests could not write by their deadline