Initial implementation of the eec-campaigns tool

Drives listmonk's real Campaign API from git-authored Markdown+frontmatter,
so broadcast/segment emails get listmonk's mature unsubscribe/bulk-send/
attachment handling instead of reimplementing it. sync only ever creates or
updates a draft (idempotent, diff-based, refuses to touch a non-draft
campaign); a pushed send/<slug> tag or manual workflow run is the only way
to actually trigger a send. Includes list-name resolution, segment_query
materialization into managed lists, content-hash-deduped attachment
uploads, and an automatic post-sync preview email.

.gitea/workflows/sync.yml

@@ -0,0 +1,22 @@
+name: Sync campaigns
+on:
+  push:
+    branches: [master]
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.23.1"
+
+      - name: Sync campaigns to listmonk
+        run: go run . sync .
+        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 }}
+          CAMPAIGNS_PREVIEW_EMAIL: ${{ secrets.CAMPAIGNS_PREVIEW_EMAIL }}