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/channel-vars.yaml

@@ -0,0 +1,48 @@
+name: channel vars
+
+on:
+  workflow_call:
+    inputs:
+      github_event_name:
+        type: string
+      github_ref_name:
+        type: string
+        required: false
+      workflow_dispatch_channel:
+        type: string
+        required: false
+
+    outputs:
+      channel:
+        description: 'Computed release channel (stable|canary)'
+        value: ${{ jobs.emit.outputs.channel }}
+      is_canary:
+        description: 'Whether this is a canary deploy (true|false)'
+        value: ${{ jobs.emit.outputs.is_canary }}
+      stack_suffix:
+        description: "Suffix for stack/image names ('' or '-canary')"
+        value: ${{ jobs.emit.outputs.stack_suffix }}
+
+jobs:
+  emit:
+    runs-on: ubuntu-latest
+    timeout-minutes: 25
+    outputs:
+      channel: ${{ steps.compute.outputs.channel }}
+      is_canary: ${{ steps.compute.outputs.is_canary }}
+      stack_suffix: ${{ steps.compute.outputs.stack_suffix }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+        with:
+          sparse-checkout: scripts/ci
+          sparse-checkout-cone-mode: false
+
+      - name: Determine channel
+        id: compute
+        shell: bash
+        run: >-
+          python3 scripts/ci/workflows/channel_vars.py
+          --event-name "${{ inputs.github_event_name }}"
+          --ref-name "${{ inputs.github_ref_name || '' }}"
+          --dispatch-channel "${{ inputs.workflow_dispatch_channel || '' }}"