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/fluxer_desktop/packaging/homebrew/fluxer.rb

@@ -0,0 +1,37 @@
+cask "fluxer" do
+  arch arm: "arm64", intel: "x64"
+
+  version "PLACEHOLDER_VERSION"
+
+  on_arm do
+    sha256 "PLACEHOLDER_SHA256_ARM64"
+  end
+  on_intel do
+    sha256 "PLACEHOLDER_SHA256_X64"
+  end
+
+  url "https://api.fluxer.app/dl/desktop/stable/darwin/#{arch}/#{version}/dmg"
+  name "Fluxer"
+  desc "Instant messaging and VoIP application"
+  homepage "https://fluxer.app"
+
+  livecheck do
+    url "https://api.fluxer.app/dl/desktop/stable/darwin/arm64/latest"
+    strategy :json do |json|
+      json["version"]
+    end
+  end
+
+  auto_updates true
+  depends_on macos: ">= :catalina"
+
+  app "Fluxer.app"
+
+  zap trash: [
+    "~/Library/Application Support/Fluxer",
+    "~/Library/Caches/app.fluxer",
+    "~/Library/Caches/app.fluxer.ShipIt",
+    "~/Library/Preferences/app.fluxer.plist",
+    "~/Library/Saved Application State/app.fluxer.savedState",
+  ]
+end