[monitoring] Add Claude OAuth token expiry monitoring + alerts

## Context

The new CLAUDE_CODE_OAUTH_TOKEN mechanism (commit 8a054752) uses
long-lived 1-year tokens minted via `claude setup-token`. Tokens don't
auto-refresh — at the 1-year mark they expire hard and the upgrade
agent stops working. We need to be told 30 days ahead, not find out
when DIUN fires and gets 401 again.

A cron rotator doesn't make sense here (tokens don't refresh, they
just expire) so we alert instead. Two spares at
`secret/claude-agent-service-spare-{1,2}` provide failover runway —
monitor covers all three.

## This change

**CronJob** (`claude-agent` ns, every 6h): reads a ConfigMap
containing `<path> → expiry_unix_timestamp` entries, pushes
`claude_oauth_token_expiry_timestamp{path="..."}` and
`claude_oauth_expiry_monitor_last_push_timestamp` to Pushgateway at
`prometheus-prometheus-pushgateway.monitoring:9091`.

**ConfigMap** generated from a Terraform local `claude_oauth_token_mint_epochs`
— source of truth for mint times. On rotation, update the map + apply.
TTL is a shared local (365d).

**PrometheusRules** (in prometheus_chart_values.tpl):
- `ClaudeOAuthTokenExpiringSoon`  — <30d, warning, for 1h
- `ClaudeOAuthTokenCritical`      — <7d,  critical, for 10m
- `ClaudeOAuthTokenMonitorStale`  — last push >48h, warning
- `ClaudeOAuthTokenMonitorNeverRun` — metric absent for 2h, warning

Alert labels include `{{ $labels.path }}` so we know which token is
expiring (primary / spare-1 / spare-2).

## Verification

```
$ kubectl -n claude-agent create job --from=cronjob/claude-oauth-expiry-monitor manual
$ curl pushgateway/metrics | grep claude_oauth_token_expiry
claude_oauth_token_expiry_timestamp{...,path="primary"} 1.808064429e+09
claude_oauth_token_expiry_timestamp{...,path="spare-1"} 1.80806428e+09
claude_oauth_token_expiry_timestamp{...,path="spare-2"} 1.808064429e+09

$ query: (claude_oauth_token_expiry_timestamp - time()) / 86400
  primary: 365.2 days
  spare-1: 365.2 days
  spare-2: 365.2 days
```

## Rotation playbook (future)

1. `kubectl run -it --rm --image=registry.viktorbarzin.me/claude-agent-service:latest tokmint -- claude setup-token`
   (or harvest via `harvest3.py` pattern in memory for headless flow)
2. `vault kv patch secret/claude-agent-service claude_oauth_token=<new>`
3. Update `claude_oauth_token_mint_epochs["primary"]` in
   `stacks/claude-agent-service/main.tf` with new unix timestamp
4. `scripts/tg apply` claude-agent-service + monitoring
5. Alert clears within 6h (next cron tick) + 1h of the
   `ClaudeOAuthTokenExpiringSoon` "for:" duration

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stacks/monitoring/modules/monitoring/prometheus_chart_values.tpl

@@ -1741,6 +1741,38 @@ serverFiles:
               severity: warning
             annotations:
               summary: "Email round-trip monitor never reported - check CronJob in mailserver namespace"
+          - alert: ClaudeOAuthTokenExpiringSoon
+            expr: (claude_oauth_token_expiry_timestamp{job="claude-oauth-expiry-monitor"} - time()) < (30 * 86400)
+            for: 1h
+            labels:
+              severity: warning
+            annotations:
+              summary: "Claude OAuth token {{ $labels.path }} expires in <30 days"
+              description: "Run `claude setup-token` to mint a new 1-year token and update the corresponding Vault path + mint_epoch in stacks/claude-agent-service/main.tf."
+          - alert: ClaudeOAuthTokenCritical
+            expr: (claude_oauth_token_expiry_timestamp{job="claude-oauth-expiry-monitor"} - time()) < (7 * 86400)
+            for: 10m
+            labels:
+              severity: critical
+            annotations:
+              summary: "Claude OAuth token {{ $labels.path }} expires in <7 days — rotate NOW"
+              description: "The long-lived CLAUDE_CODE_OAUTH_TOKEN is within 1 week of expiry. Automated upgrades will break when it expires. Harvest via `claude setup-token` and update Vault + TF."
+          - alert: ClaudeOAuthTokenMonitorStale
+            expr: (time() - claude_oauth_expiry_monitor_last_push_timestamp) > (48 * 3600)
+            for: 10m
+            labels:
+              severity: warning
+            annotations:
+              summary: "Claude OAuth expiry monitor hasn't pushed in >48h"
+              description: "CronJob claude-oauth-expiry-monitor in claude-agent ns isn't running. Check `kubectl -n claude-agent get cronjob claude-oauth-expiry-monitor`."
+          - alert: ClaudeOAuthTokenMonitorNeverRun
+            expr: absent(claude_oauth_expiry_monitor_last_push_timestamp)
+            for: 2h
+            labels:
+              severity: warning
+            annotations:
+              summary: "Claude OAuth expiry monitor has never pushed — CronJob not running"
+              description: "Expected `claude_oauth_expiry_monitor_last_push_timestamp` to appear once the CronJob runs. Check the CronJob in claude-agent namespace."
           - alert: HackmdDown
             expr: (kube_deployment_status_replicas_available{namespace="hackmd"} or on() vector(0)) < 1
             for: 5m