secrets: rotate + de-git Resend key; source from out-of-band Secret/resend-api

Resend send-scoped key was committed in plaintext in manifests/hub.yaml (ConfigMap)
and manifests/felhom.secret.yaml. Rotated to a new key and removed from git.

- hub: new RESEND_API_KEY env override (cmd/hub/main.go), mirrors REGISTRY_TOKEN;
  ConfigMap resend_api_key now an empty placeholder; Deployment injects from
  Secret/resend-api. Image 0.17.0.
- contact-mailer: secretKeyRef repointed contact-mailer-config -> resend-api.
- felhom.secret.yaml: contact-mailer-config Secret removed; healthchecks
  EMAIL_HOST_PASSWORD blanked (workload not deployed).
- documentation/runbooks/secrets.md: out-of-band secret model + create/rotate steps.

Secret/resend-api is created imperatively out-of-band and is NOT committed.
No secret value appears in this repo.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

manifests/contact-mailer.yaml

@@ -10,10 +10,10 @@
 #      # Option B: Import directly into k3s (single node):
 #      #   docker save contact-mailer:latest | sudo k3s ctr images import -
 #
-#   2. Create the Secret with your Resend API key:
-#      kubectl create secret generic contact-mailer-config \
-#        --namespace=felhom-system \
-#        --from-literal=RESEND_API_KEY='re_xxxxxxxxxxxx'
+#   2. The Resend API key comes from the out-of-band Secret/resend-api (NOT committed).
+#      Create it per documentation/runbooks/secrets.md (key sourced from the out-of-band store):
+#        kubectl create secret generic resend-api -n felhom-system \
+#          --from-literal=RESEND_API_KEY="$RESEND_API"
 #
 #   3. Apply this manifest:
 #      kubectl apply -f contact-mailer.yaml
@@ -56,10 +56,12 @@ spec:
           ports:
             - containerPort: 8080
           env:
+            # Resend API key — injected from the shared out-of-band Secret/resend-api (NOT committed).
+            # See documentation/runbooks/secrets.md.
             - name: RESEND_API_KEY
               valueFrom:
                 secretKeyRef:
-                  name: contact-mailer-config
+                  name: resend-api
                   key: RESEND_API_KEY
             - name: FROM_EMAIL
               value: "Felhom.eu <noreply@felhom.eu>"