feat(instance-proxy): add HTTP proxy for tool instances

Add API proxy endpoint so users can access running tool instances
through the backend API instead of internal Docker network.

Backend:
- Add container_name field to ToolInstance model
- Create /instances/{id}/proxy/{path:path} endpoint with ownership checks
- Proxy HTTP requests to containers via docker network using container names
- Support all HTTP methods (GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS)
- Store proxy URL in instance.url instead of localhost
- Add Alembic migration 0007 for container_name column
- Add get_container_name() utility to docker.py

Frontend:
- Update Open button to use full proxy URL (API_BASE_URL + instance.url)

Closes instance-proxy OpenSpec change.

apps/web/src/components/instance-list.tsx

@@ -12,6 +12,8 @@ import {
 } from "../api/sessions";
 import type { ToolType } from "../api/tool_types";
 
+const API_BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
+
 interface InstanceListProps {
   projectId: string;
   repoId: string;
@@ -147,7 +149,7 @@ export const InstanceList = ({ projectId, repoId, toolTypes }: InstanceListProps
               <div className="instance-actions">
                 {instance.status === "running" && instance.url && (
                   <a
-                    href={instance.url}
+                    href={`${API_BASE_URL}${instance.url}`}
                     target="_blank"
                     rel="noopener noreferrer"
                     className="secondary-button small"