acme: migrate from step-ca to OpenBao PKI

Switch all ACME certificate issuance from step-ca (ca.home.2rjus.net)
to OpenBao PKI (vault.home.2rjus.net:8200/v1/pki_int/acme/directory).

- Update default ACME server in system/acme.nix
- Update Caddy acme_ca in http-proxy and nix-cache services
- Remove labmon service from monitoring01 (step-ca monitoring)
- Remove labmon scrape target and certificate_rules alerts
- Remove alloy.nix (only used for labmon profiling)
- Add docs/plans/cert-monitoring.md for future cert monitoring needs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/monitoring/rules.yml

@@ -338,40 +338,6 @@ groups:
         annotations:
           summary: "Pyroscope service not running on {{ $labels.instance }}"
           description: "Pyroscope service not running on {{ $labels.instance }}"
-  - name: certificate_rules
-    rules:
-      - alert: certificate_expiring_soon
-        expr: labmon_tlsconmon_certificate_seconds_left{address!="ca.home.2rjus.net:443"} < 86400
-        for: 5m
-        labels:
-          severity: warning
-        annotations:
-          summary: "TLS certificate expiring soon for {{ $labels.instance }}"
-          description: "TLS certificate for {{ $labels.address }} is expiring within 24 hours."
-      - alert: step_ca_serving_cert_expiring
-        expr: labmon_tlsconmon_certificate_seconds_left{address="ca.home.2rjus.net:443"} < 3600
-        for: 5m
-        labels:
-          severity: critical
-        annotations:
-          summary: "Step-CA serving certificate expiring"
-          description: "The step-ca serving certificate (24h auto-renewed) has less than 1 hour of validity left. Renewal may have failed."
-      - alert: certificate_check_error
-        expr: labmon_tlsconmon_certificate_check_error == 1
-        for: 5m
-        labels:
-          severity: warning
-        annotations:
-          summary: "Error checking certificate for {{ $labels.address }}"
-          description: "Certificate check is failing for {{ $labels.address }} on {{ $labels.instance }}."
-      - alert: step_ca_certificate_expiring
-        expr: labmon_stepmon_certificate_seconds_left < 3600
-        for: 5m
-        labels:
-          severity: critical
-        annotations:
-          summary: "Step-CA certificate expiring for {{ $labels.instance }}"
-          description: "Step-CA certificate is expiring within 1 hour on {{ $labels.instance }}."
   - name: proxmox_rules
     rules:
       - alert: pve_node_down