infra/docs: Docker Hub release pipeline, versioning, README overhaul

Reference prebuilt temetro/temetro-{backend,frontend} images in docker-compose
(with a build fallback) and stop baking the frontend API URL. Add a
tag-triggered GitHub Actions release workflow that pushes both images and cuts
a GitHub Release, plus RELEASING.md, CHANGELOG.md, and a root version. Overhaul
the root and frontend READMEs (screenshot, features, prebuilt-image quickstart,
LAN access, updating).

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

.github/workflows/release.yml

@@ -0,0 +1,67 @@
+# Builds and publishes the temetro Docker images and cuts a GitHub Release.
+#
+# Trigger: push a semver tag, e.g.
+#   git tag v0.1.0 && git push origin v0.1.0
+#
+# The frontend image bakes NO API URL — it resolves the backend from the host
+# the browser uses at runtime — so one published image works for every clinic.
+#
+# Required repository secrets (Settings → Secrets and variables → Actions):
+#   DOCKERHUB_USERNAME  — a Docker Hub account with push access to `temetro`
+#   DOCKERHUB_TOKEN     — a Docker Hub access token for that account
+# Without them the build/login step fails; the workflow is otherwise inert.
+name: release
+
+on:
+  push:
+    tags:
+      - "v*.*.*"
+
+permissions:
+  contents: write # create the GitHub Release (the update check reads this)
+
+env:
+  REGISTRY_NAMESPACE: temetro
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Derive version from tag
+        id: meta
+        run: echo "version=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Build & push backend
+        uses: docker/build-push-action@v6
+        with:
+          context: ./backend
+          push: true
+          tags: |
+            ${{ env.REGISTRY_NAMESPACE }}/temetro-backend:${{ steps.meta.outputs.version }}
+            ${{ env.REGISTRY_NAMESPACE }}/temetro-backend:latest
+
+      - name: Build & push frontend
+        uses: docker/build-push-action@v6
+        with:
+          context: ./frontend
+          push: true
+          tags: |
+            ${{ env.REGISTRY_NAMESPACE }}/temetro-frontend:${{ steps.meta.outputs.version }}
+            ${{ env.REGISTRY_NAMESPACE }}/temetro-frontend:latest
+
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@v2
+        with:
+          generate_release_notes: true