feat: add fluxer upstream source and self-hosting documentation

- Clone of github.com/fluxerapp/fluxer (official upstream)
- SELF_HOSTING.md: full VM rebuild procedure, architecture overview,
  service reference, step-by-step setup, troubleshooting, seattle reference
- dev/.env.example: all env vars with secrets redacted and generation instructions
- dev/livekit.yaml: LiveKit config template with placeholder keys
- fluxer-seattle/: existing seattle deployment setup scripts

fluxer/.github/workflows/migrate-cassandra.yaml

@@ -0,0 +1,67 @@
+name: migrate cassandra
+
+on:
+  push:
+    branches:
+      - canary
+    paths:
+      - fluxer_devops/cassandra/migrations/**/*.cql
+  workflow_dispatch:
+
+concurrency:
+  group: migrate-cassandra-prod
+  cancel-in-progress: false
+
+permissions:
+  contents: read
+
+jobs:
+  migrate:
+    name: Run database migrations
+    runs-on: blacksmith-8vcpu-ubuntu-2404
+    timeout-minutes: 25
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 24
+          cache: pnpm
+          cache-dependency-path: pnpm-lock.yaml
+
+      - name: Install dependencies
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step install_dependencies
+
+      - name: Validate migrations
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step validate_migrations
+
+      - name: Set up SSH agent
+        uses: webfactory/ssh-agent@v0.9.1
+        with:
+          ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY_SERVER }}
+
+      - name: Add server to known hosts
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step add_known_hosts --server-ip ${{ secrets.SERVER_IP }}
+
+      - name: Set up SSH tunnel for Cassandra
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step setup_tunnel --server-user ${{ secrets.SERVER_USER }} --server-ip ${{ secrets.SERVER_IP }}
+
+      - name: Test Cassandra connection
+        env:
+          CASSANDRA_USERNAME: ${{ secrets.CASSANDRA_USERNAME }}
+          CASSANDRA_PASSWORD: ${{ secrets.CASSANDRA_PASSWORD }}
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step test_connection
+
+      - name: Run migrations
+        env:
+          CASSANDRA_USERNAME: ${{ secrets.CASSANDRA_USERNAME }}
+          CASSANDRA_PASSWORD: ${{ secrets.CASSANDRA_PASSWORD }}
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step run_migrations
+
+      - name: Close SSH tunnel
+        if: always()
+        run: python3 scripts/ci/workflows/migrate_cassandra.py --step close_tunnel