Default the Web App to :8090 (BFF) in the API Server

Point the API Server's Web App references at the production BFF port
instead of the Vite dev port (5173): the WEBAPP_URL default probed by
/api/status, and the panel's Web App URL / allowed-origins placeholders.
Update .env.example and the README env table to match. Rebuild the
embedded admin panel dist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API Server/internal/config/config.go

@@ -47,7 +47,7 @@ func Load() Config {
 	return Config{
 		Addr:                    normalizeAddr(firstEnv("API_ADDR", "PORT"), ":8080"),
 		PocketBaseURL:           strings.TrimRight(firstEnvOr("http://10.2.1.10:8027", "POCKETBASE_URL", "PB_URL"), "/"),
-		WebAppURL:               strings.TrimRight(getenv("WEBAPP_URL", "http://localhost:5173"), "/"),
+		WebAppURL:               strings.TrimRight(getenv("WEBAPP_URL", "http://localhost:8090"), "/"),
 		AllowOrigins:            splitCSV(firstEnvOr("*", "CORS_ALLOW_ORIGINS", "CORS_ORIGINS")),
 		UsersCollection:         getenv("AUTH_USERS_COLLECTION", "users"),
 		PluginsFile:             getenv("PLUGINS_FILE", "plugins.json"),