feat: owner accounts and in-app platform settings

- First registered account becomes admin (users.is_admin, backfilled
  to earliest account); better-auth mirror path does the same
- Public registration closes after bootstrap; admin can reopen it via
  GET/PUT /api/v1/settings (admin-gated) or SIGNUP_ENABLED env
- In-app settings override env vars; secrets stored in app_settings,
  token values never returned by the API
- Saving a Cloudflare Tunnel token pulls and runs a managed
  cloudflared sidecar on the backend's networks; clearing removes it;
  startup restores it from saved settings
- group_add DOCKER_GID grants the non-root backend user Docker socket
  access; install.sh probes the in-container socket gid
- Settings page gains an owner-only Platform section (registration
  toggle, tunnel token, link to Cloudflare dashboard); sign-up page
  stays reachable while registration is open
- OpenAPI spec + generated types updated; manual user creation now
  requires admin

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/routes.go

@@ -35,6 +35,8 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 		dockerClient = client
 		buildManager = build.NewBuildManager("/tmp/containr-builds", dockerClient)
 		deploymentEngine = deployment.NewDeploymentEngine(buildManager, dockerClient)
+		// Restore managed sidecars (e.g. cloudflared) from saved settings.
+		syncManagedContainers(dockerClient, db)
 	}
 
 	// Initialize build handler
@@ -184,6 +186,10 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			protected.PUT("/user/profile", handleUpdateProfile)
 			protected.POST("/users", handleCreateUser)
 
+			// Platform settings (admin-gated inside handlers)
+			protected.GET("/settings", handleGetSettings)
+			protected.PUT("/settings", handleUpdateSettings)
+
 			// Project routes
 			protected.GET("/projects", handleGetProjects)
 			protected.POST("/projects", handleCreateProject)