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/devenv.yaml

@@ -0,0 +1,10 @@
+# yaml-language-server: $schema=https://devenv.sh/devenv.schema.json
+inputs:
+  devenv:
+    url: github:cachix/devenv/v1.11.2
+  nixpkgs:
+    url: github:cachix/devenv-nixpkgs/rolling
+  rust-overlay:
+    url: github:oxalica/rust-overlay
+    overlays:
+      - default