feat: add reverse proxy support with X-Forwarded-Host header handling for agent installer URL generation

Add external_base_url helper to detect base URL from X-Forwarded-Host and X-Forwarded-Proto headers with fallback to request.base_url, replace hardcoded request.base_url usage in node_agent_install, node_agent_install_info, and public_node_agent_install endpoints to support reverse proxy deployments, update nginx config to pass X-Forwarded-Host header and use $http_host instead of $host for proper hostname forw

backend/app/api/v1/router.py

@@ -658,6 +658,14 @@ def node_agent_payload(node: Node, agent: NodeAgent | None) -> dict:
     }
 
 
+def external_base_url(request: Request) -> str:
+    forwarded_host = request.headers.get("x-forwarded-host") or request.headers.get("host")
+    forwarded_proto = request.headers.get("x-forwarded-proto") or request.url.scheme
+    if forwarded_host:
+        return f"{forwarded_proto}://{forwarded_host}".rstrip("/")
+    return str(request.base_url).rstrip("/")
+
+
 @api_router.get("/nodes/agents", response_model=list[NodeWithAgentRead])
 def nodes_with_agents(_: CurrentUser, db: Session = Depends(get_db)) -> list[dict]:
     agents = {agent.node_id: agent for agent in db.scalars(select(NodeAgent)).all()}
@@ -742,7 +750,7 @@ def node_agent_install(node_id: str, request: Request, user: CurrentUser, db: Se
     agent.install_count = (agent.install_count or 0) + 1
     token = create_token(node.id, "agent", timedelta(days=365), {"node_name": node.name})
     write_audit(db, action="agent.install.generated", object_type="node", object_id=node.id, user_id=user.id)
-    base_url = str(request.base_url).rstrip("/")
+    base_url = external_base_url(request)
     return agent_install_script(base_url, token, node)
 
 
@@ -758,7 +766,7 @@ def node_agent_install_info(node_id: str, request: Request, user: CurrentUser, d
     agent.install_count = (agent.install_count or 0) + 1
     install_token = create_token(node.id, "agent-install", timedelta(days=7), {"node_name": node.name})
     write_audit(db, action="agent.install.generated", object_type="node", object_id=node.id, user_id=user.id)
-    base_url = str(request.base_url).rstrip("/")
+    base_url = external_base_url(request)
     install_url = f"{base_url}/api/v1/agents/install/{node.id}?token={install_token}"
     return {
         "node_id": node.id,
@@ -779,7 +787,7 @@ def public_node_agent_install(node_id: str, token: str, request: Request, db: Se
     if not node:
         raise HTTPException(status_code=404, detail="Node not found")
     agent_token = create_token(node.id, "agent", timedelta(days=365), {"node_name": node.name})
-    base_url = str(request.base_url).rstrip("/")
+    base_url = external_base_url(request)
     return agent_install_script(base_url, agent_token, node)