feat(deploy): three deployment modes + prebuilt multi-arch image for robust self-hosting

Make Backspace self-hostable in any homelab environment, not just a clean host
that owns ports 80/443.

install.sh is now mode-aware and auto-detects which fits:
  - allinone (default): bundled Caddy + auto-HTTPS — unchanged behavior
  - proxy: behind your own reverse proxy (nginx / Traefik / Caddy / Nginx Proxy
    Manager / SWAG) — app published on 127.0.0.1:APP_PORT, no bundled Caddy,
    prints paste-ready proxy snippets
  - tunnel: behind a tunnel (Cloudflare / Tailscale) — same, plus a 90MB upload
    cap (under Cloudflare's 100MB body limit) and voice force-disabled (WebRTC
    over UDP can't traverse a tunnel)

Port detection is Docker-aware (consults `docker ps` published ports, not just
`ss`), so a host whose proxy already owns 80/443 via iptables DNAT — with no
listening socket for `ss` to see — is correctly detected as "taken" instead of
dead-ending.

docker-compose.proxy.yml is a small overlay, layered via COMPOSE_FILE (written
into .env so no `-f` flags are ever needed), that publishes the loopback port and
parks Caddy in an inert profile. The base compose file is untouched, so All-in-One
behaves exactly as before.

Prebuilt image: .github/workflows/docker-publish.yml builds and pushes a
multi-arch (linux/amd64 + linux/arm64) image to ghcr.io/thezwiss/backspace on
release tags (and manual dispatch), so weak/ARM hosts skip the ~1.6GB local build
(the Vite build OOMs small ARM boxes). install.sh and docker-compose.yml default
to pulling it, fall back to an image already present on the host, and finally to a
from-source build — AGPL §13 commit stamping preserved on every path. Kept
deliberately separate from the desktop-installer workflow (release.yml).

Docs: README gains a "Deployment modes" section (all three modes, nginx / Caddy /
Traefik snippets, GUI-proxy field-by-field, cloudflared ingress, the update path,
and voice-per-mode caveats); docs/systems/deployment.md updated to match.

Verified live on a throwaway VM: proxy + all-in-one end-to-end through install.sh
(with a real Let's Encrypt cert), tunnel config generation, loopback-only binding,
and the local-image fallback path.

.github/workflows/docker-publish.yml

@@ -0,0 +1,92 @@
+name: Publish Container Image
+
+# Builds and publishes the Backspace application image to the GitHub Container
+# Registry (GHCR) as a multi-architecture (linux/amd64 + linux/arm64) image, so
+# self-hosters — including weak/ARM boxes like a Raspberry Pi — can `docker pull`
+# a prebuilt image instead of building the ~1.6 GB image locally (the Vite build
+# OOMs small ARM hosts). install.sh and docker-compose.yml default to pulling
+# this image, with a from-source build as the fallback.
+#
+# This is intentionally SEPARATE from the desktop-installer workflow
+# (release.yml) — they share the `v*` tag trigger but build entirely different
+# artifacts and must not be entangled.
+
+on:
+  push:
+    tags:
+      - 'v*'
+  # Allow a manual rebuild/publish (e.g. to (re)publish `latest` or a moving tag
+  # without cutting a new release).
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Extra tag to publish (optional, e.g. "edge")'
+        required: false
+        default: ''
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      # The runtime image bakes the git commit for the AGPL-3.0 § 13 source
+      # offer (config.commit → GET /api/instance/info). The .git dir is not in
+      # the build context (.dockerignore), so resolve the short SHA here and feed
+      # it to the build as a --build-arg, matching install.sh / deploy.sh.
+      - name: Resolve build metadata
+        id: meta_commit
+        run: echo "commit=$(git rev-parse --short HEAD)" >> "$GITHUB_OUTPUT"
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Derive image tags and labels
+        id: docker_meta
+        uses: docker/metadata-action@v5
+        with:
+          # github.repository is "TheZwiss/backspace"; metadata-action lowercases
+          # it → ghcr.io/thezwiss/backspace (GHCR requires lowercase).
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
+            type=sha
+            type=raw,value=${{ github.event.inputs.tag }},enable=${{ github.event.inputs.tag != '' }}
+          labels: |
+            org.opencontainers.image.title=Backspace
+            org.opencontainers.image.description=Self-hosted Discord alternative — text, voice, video, and federation.
+            org.opencontainers.image.source=https://github.com/TheZwiss/backspace
+            org.opencontainers.image.licenses=AGPL-3.0-only
+            org.opencontainers.image.revision=${{ github.sha }}
+
+      - name: Build and push (linux/amd64, linux/arm64)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.docker_meta.outputs.tags }}
+          labels: ${{ steps.docker_meta.outputs.labels }}
+          build-args: |
+            BACKSPACE_COMMIT=${{ steps.meta_commit.outputs.commit }}
+          # Cache multi-arch layers across runs via the GitHub Actions cache to
+          # keep the ~1.6 GB build from re-running cold every release.
+          cache-from: type=gha
+          cache-to: type=gha,mode=max