Add template-as-code sync (campaigns template NAME PATH)

Answers "can the unsubscribe template be added via git+API too": commit
listmonk's own stock campaign template (already unsubscribe-capable) as
email-templates/campaign.html, add a small find-or-create-or-update
Template client (confirmed against knadh/listmonk's actual model/handlers),
a new `template` subcommand, and a manual-dispatch-only workflow to push it
in as the default campaign template. Unlike campaign sync there's no
draft/live status to protect, so this is always a safe overwrite.

.gitea/workflows/sync-template.yml

@@ -0,0 +1,20 @@
+name: Sync default campaign template
+on:
+  workflow_dispatch: {}
+
+jobs:
+  sync-template:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.23.1"
+
+      - name: Sync template to listmonk
+        run: go run . template "eec-campaigns default" email-templates/campaign.html
+        env:
+          LISTMONK_BASE_URL: ${{ secrets.CAMPAIGNS_LISTMONK_BASE_URL }}
+          LISTMONK_API_USER: ${{ secrets.CAMPAIGNS_LISTMONK_API_USER }}
+          LISTMONK_API_TOKEN: ${{ secrets.CAMPAIGNS_LISTMONK_API_TOKEN }}