docs: migrate all diagrams from PlantUML to Mermaid

- Replace section 13 of ARCHITECTURE.md with 9 inline Mermaid diagrams
  (component, auth sequence, dashboard SSE sequence, polling sequence,
  server class, data model, UI state, poller state, matching flowchart)
- Diagrams render natively in Gitea/GitHub — no CI job required
- Delete docs/diagrams/*.puml (all 9 files)
- Delete .gitea/workflows/render-diagrams.yml
- Update CI/CD table note and ToC entry

.gitea/workflows/render-diagrams.yml

@@ -1,42 +0,0 @@
-name: Render PlantUML Diagrams
-
-on:
-  push:
-    branches: ["main", "develop", "release/**"]
-    paths:
-      - "docs/diagrams/**.puml"
-
-jobs:
-  render:
-    name: Render .puml → .png
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-        with:
-          token: ${{ secrets.BOT_TOKEN }}
-
-      - name: Install Java & Graphviz
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install -y --no-install-recommends default-jre-headless graphviz
-
-      - name: Download PlantUML jar
-        run: |
-          curl -sSL -o /usr/local/bin/plantuml.jar \
-            https://github.com/plantuml/plantuml/releases/download/v1.2024.6/plantuml-1.2024.6.jar
-
-      - name: Render diagrams
-        run: |
-          java -jar /usr/local/bin/plantuml.jar -tpng -o . docs/diagrams/*.puml
-
-      - name: Commit rendered PNGs
-        run: |
-          git config user.name  "gitea-actions[bot]"
-          git config user.email "gitea-actions[bot]@i3omb.com"
-          git add docs/diagrams/*.png
-          if git diff --cached --quiet; then
-            echo "No diagram changes to commit."
-          else
-            git commit -m "ci: render PlantUML diagrams [skip ci]"
-            git push
-          fi