fix(fire-planner): grafana fire-planner-pg datasource survives pw rotation

The fire-planner-pg Grafana datasource baked the rotating fire_planner DB
password into its provisioning ConfigMap at terraform plan-time, so on every
7-day static-role rotation the password went stale and ALL fire-planner-pg
dashboards (fire-planner, cost-of-living, and the new wealth FIRE Countdown)
silently failed with "password authentication failed for user fire_planner"
until the next stack apply.

Switch to the same live-env pattern wealth-pg / payslips-pg already use:
- new ExternalSecret grafana-fire-planner-pg-creds (monitoring ns, Reloader
  match) mirrors the rotating Vault static-creds/pg-fire-planner password
- datasource ConfigMap now references $__env{FIRE_PLANNER_PG_PASSWORD}
- Grafana mounts it via envFromSecrets; reloader (auto) restarts Grafana on
  rotation so the provisioned datasource never goes stale

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

stacks/monitoring/modules/monitoring/grafana_chart_values.yaml

@@ -86,6 +86,8 @@ envFromSecrets:
     optional: true
   - name: grafana-job-hunter-pg-creds
     optional: true
+  - name: grafana-fire-planner-pg-creds
+    optional: true
 env:
   GF_SERVER_ROOT_URL: https://grafana.viktorbarzin.me