ai: persist FlowAgent MSAL token cache via mcpo bind mount

Adds the missing bind mount for FlowAgent's delegated-user MSAL disk
token cache:

    /volume1/docker/mcpo/flowagent-auth:/app/flowagent-auth

Confirmed via `docker service inspect ai_mcpo --format '{{json .Spec.TaskTemplate.ContainerSpec.Mounts}}'`-equivalent review of the current
compose that mcpo's mounts were only: ssh_keys, config.json, mcpo_data,
uv-cache -- flowagent-auth (set as HOME for the flowagent subprocess via
config.json.template's env.HOME=/app/flowagent-auth) was NOT persisted,
so any successful delegated-user MSAL sign-in (list_connections /
create_connection / resolve_refs / pick_or_create_connection /
resolve_params -- the tools that use MsalTokenProvider, see companion
homelab/mcp-config PR #9) would be silently lost on every mcpo restart,
forcing a fresh interactive sign-in every time.

Host dir already exists (pre-created on shared CephFS, identical across
all three docker LXCs):
    mkdir -p /volume1/docker/mcpo/flowagent-auth

This does NOT change the flowagent image tag/pin -- that stays
git.bryanmail.net/homelab/flowagent-mcpo:c5b22618404a pending the
companion mcp-config PR merging and its Woodpecker build producing a new
commit-sha tag to bump to.

No other changes to ai.yaml in this commit -- diff is mount-list addition
only.

ai/ai.yaml

@@ -141,6 +141,7 @@ services:
     - /volume1/docker/mcpo/data:/mcpo_data
     - /volume1/docker/cronicle/ssh_keys:/app/ssh_keys:ro
     - /volume1/docker/mcpo/uv-cache:/app/uv-cache
+    - /volume1/docker/mcpo/flowagent-auth:/app/flowagent-auth
     secrets:
     - source: flowagent_azure_client_id
       target: flowagent_azure_client_id