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/api/src/main.py

@@ -15,6 +15,7 @@ from src.api.health import router as health_router
 from src.api.projects import router as projects_router
 from src.api.ssh_keys import router as ssh_keys_router
 from src.api.terminal import router as terminal_router
+from src.api.instance_proxy import router as instance_proxy_router
 from src.api.tool_instances import router as tool_instances_router
 from src.api.tool_instances import sessions_router
 from src.api.tool_types import router as tool_types_router
@@ -184,5 +185,6 @@ app.include_router(user_config_router)
 app.include_router(tool_types_router)
 app.include_router(tool_instances_router)
 app.include_router(sessions_router)
+app.include_router(instance_proxy_router)
 app.include_router(terminal_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")