Add installer, CI, ARM build and test updates

Introduce a one‑line Docker installer and environment examples, add CI and docker validation workflows, and update docs and tests.

Key changes:
- Add docker/install.sh installer that bootstraps /opt/synapsis, downloads compose files, optionally installs Docker, and generates secrets.
- Add top-level .env.example and docker/.env.example entries for shared storage and local development.
- Add GitHub Actions CI (ci.yml) with type checks, targeted vitest runs, build and docker-compose validation.
- Update existing docker workflow to set up QEMU and build multi‑arch images (amd64, arm64).
- Update README and docker/README to use the installer, point to the new repo, adjust local setup instructions, and note shared S3 env vars.
- Update docker-compose comments and include shared S3 env variables and defaults.
- Update .gitignore to ignore site-work mirrors.
- Tests: expand supported bot source types (add brave_news, youtube), increase POST_MAX_LENGTH from 400 to 600, add minimal user handle in mention handler tests, and add mocks & test adjustments in scheduler tests to keep them unit-scoped.

These changes simplify installation, add CI coverage (including Docker config validation), enable multi‑arch builds, and align tests with expanded bot source/validation behavior.

.github/workflows/docker.yml

@@ -29,6 +29,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -61,4 +64,4 @@ jobs:
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           no-cache: true
-          platforms: linux/amd64
+          platforms: linux/amd64,linux/arm64