fix(storage): emit origin-relative object URLs

Build signed object URLs as path-only (/api/objects/...) instead of
prefixing APP_BASE_URL, so media loads from whatever host or reverse
proxy served the app. Remove the now-unused base URL from the signer and
drop the APP_BASE_URL / Server.BaseURL config.

backend/internal/config/config.go

@@ -39,8 +39,7 @@ type Auth struct {
 }
 
 type Server struct {
-	Addr    string
-	BaseURL string
+	Addr string
 }
 
 type SSE struct {
@@ -227,8 +226,7 @@ func Load() (*Config, error) {
 			RefreshTTL: refreshTTL,
 		},
 		Server: Server{
-			Addr:    getEnv("HTTP_ADDR", ":8080"),
-			BaseURL: getEnv("APP_BASE_URL", "http://localhost:8080"),
+			Addr: getEnv("HTTP_ADDR", ":8080"),
 		},
 		SSE: SSE{
 			HeartbeatInterval: heartbeat,