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/winget/Fluxer.Fluxer.installer.yaml

@@ -0,0 +1,19 @@
+PackageIdentifier: Fluxer.Fluxer
+PackageVersion: PLACEHOLDER_VERSION
+MinimumOSVersion: 10.0.17763.0
+InstallerType: nullsoft
+Scope: user
+InstallModes:
+  - interactive
+  - silent
+  - silentWithProgress
+UpgradeBehavior: install
+Installers:
+  - Architecture: x64
+    InstallerUrl: https://api.fluxer.app/dl/desktop/stable/win32/x64/PLACEHOLDER_VERSION/setup
+    InstallerSha256: PLACEHOLDER_SHA256_X64
+  - Architecture: arm64
+    InstallerUrl: https://api.fluxer.app/dl/desktop/stable/win32/arm64/PLACEHOLDER_VERSION/setup
+    InstallerSha256: PLACEHOLDER_SHA256_ARM64
+ManifestType: installer
+ManifestVersion: 1.6.0