Wire up image handler endpoint with service orchestration

- Add image proxy config options (signing_key, whitelist_hosts, allow_http)
- Create Service to orchestrate cache, fetcher, and processor
- Initialize image service in handlers OnStart hook
- Implement HandleImage with URL parsing, signature validation, cache
- Implement HandleRobotsTxt for search engine prevention
- Parse query params for signature, quality, and fit mode

internal/imgcache/module.go

@@ -0,0 +1,10 @@
+package imgcache
+
+import "time"
+
+// CacheConfig defaults.
+const (
+	DefaultCacheTTL     = 24 * time.Hour
+	DefaultNegativeTTL  = 5 * time.Minute
+	DefaultHotCacheSize = 1000
+)