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/.vscode/extensions.json

@@ -0,0 +1,10 @@
+{
+	"recommendations": [
+		"TypeScriptTeam.native-preview",
+		"biomejs.biome",
+		"clinyong.vscode-css-modules",
+		"pgourlain.erlang",
+		"golang.go",
+		"rust-lang.rust-analyzer"
+	]
+}