Phase 23: per-stack secrets & configs (compose file-based), local + agent (0.29.0)

Manage Docker secrets and configs per stack from a new Secrets tab on Stack/
RemoteStackDetail. Content is stored as files inside the stack dir
(.secrets/<name>, .configs/<name>; dir 0700 / file 0600) and referenced from the
compose file with relative `file:` paths, so the daemon reads them without any
HOST_ROOT_PREFIX dependency. Content is write-only — the API only ever returns
metadata (name, kind, size).

- secret_service: write/delete/list (metadata only)/exists/rel_path/attach/detach;
  name validation rejects traversal/hidden/separators, content capped at 1 MiB.
- compose_edit_service: add/remove secret and config (top-level defs pruned when
  no service still references them).
- routers/secrets.py (admin-only, audit secret.*) + agent endpoints + multi-host
  proxy (audit agent.secret.*).
- Frontend SecretsPanel (create/list/delete + per-row attach/detach to a service;
  config rows take a mount target), agentId-aware for remote stacks.

Verified: name-sandbox + perms + metadata-only listing unit-tested; compose
add/remove round-trips to clean YAML; py_compile + backend/agent/frontend image
builds + route smoke-test (local/agent/proxy). Live exec check (/run/secrets/<name>
on a deployed stack) and swarm path are hardware-verify debt (swarm dropped: A).

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

backend/routers/agents.py

@@ -19,6 +19,7 @@ from models.stack import StackCreate, StackUpdate
 from models.user import User
 from routers.files import NameBody, RenameBody, TransferBody, WriteBody
 from routers.networks import ContainerRef, NetworkCreate
+from routers.secrets import AttachBody, DetachBody, SecretWrite
 from models.auto_update import AutoUpdateRead, AutoUpdateWrite
 from services import (
     agent_service,
@@ -1039,3 +1040,92 @@ async def agent_run_auto_update(
     await auto_update_service.run_policy(session, policy)
     session.refresh(policy)
     return auto_update_service.to_read(session, policy, stack_id, agent_id)
+
+
+# --------------------------------------------------------------------------- #
+# Secrets & configs (proxied) — remote stacks
+# --------------------------------------------------------------------------- #
+
+
+@router.get("/{agent_id}/stacks/{stack_id}/secrets")
+async def agent_list_secrets(
+    agent_id: int,
+    stack_id: str,
+    session: Session = Depends(get_session),
+    _user: User = Depends(require_admin),
+) -> list:
+    agent = _get_or_404(session, agent_id)
+    return await _proxy(session, agent, "GET", f"/agent/stacks/{stack_id}/secrets") or []
+
+
+@router.put("/{agent_id}/stacks/{stack_id}/secrets")
+async def agent_write_secret(
+    agent_id: int,
+    stack_id: str,
+    body: SecretWrite,
+    request: Request,
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(session, agent, "PUT", f"/agent/stacks/{stack_id}/secrets", json=body.model_dump())
+    audit_service.record(
+        session, user=user.username, action="agent.secret.write",
+        target=f"{agent.name}/{stack_id}/{body.kind}/{body.name}", ip=_ip(request),
+    )
+    return result
+
+
+@router.delete("/{agent_id}/stacks/{stack_id}/secrets/{kind}/{name}")
+async def agent_delete_secret(
+    agent_id: int,
+    stack_id: str,
+    kind: str,
+    name: str,
+    request: Request,
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(session, agent, "DELETE", f"/agent/stacks/{stack_id}/secrets/{kind}/{name}")
+    audit_service.record(
+        session, user=user.username, action="agent.secret.delete",
+        target=f"{agent.name}/{stack_id}/{kind}/{name}", ip=_ip(request),
+    )
+    return result
+
+
+@router.post("/{agent_id}/stacks/{stack_id}/secrets/attach")
+async def agent_attach_secret(
+    agent_id: int,
+    stack_id: str,
+    body: AttachBody,
+    request: Request,
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(session, agent, "POST", f"/agent/stacks/{stack_id}/secrets/attach", json=body.model_dump())
+    audit_service.record(
+        session, user=user.username, action="agent.secret.attach",
+        target=f"{agent.name}/{stack_id}/{body.kind}/{body.name}->{body.service}", ip=_ip(request),
+    )
+    return result
+
+
+@router.post("/{agent_id}/stacks/{stack_id}/secrets/detach")
+async def agent_detach_secret(
+    agent_id: int,
+    stack_id: str,
+    body: DetachBody,
+    request: Request,
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(session, agent, "POST", f"/agent/stacks/{stack_id}/secrets/detach", json=body.model_dump())
+    audit_service.record(
+        session, user=user.username, action="agent.secret.detach",
+        target=f"{agent.name}/{stack_id}/{body.kind}/{body.name}->{body.service}", ip=_ip(request),
+    )
+    return result