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_devops/nats_core/compose.yaml

@@ -0,0 +1,29 @@
+services:
+  nats_core:
+    image: nats:2-alpine
+    hostname: nats_core
+    command: ['-c', '/etc/nats/nats.conf']
+    env_file:
+      - /etc/fluxer/nats_core.env
+    volumes:
+      - ./nats.conf:/etc/nats/nats.conf:ro
+    networks:
+      - fluxer-shared
+    ports:
+      - '4222:4222'
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
+        delay: 10s
+        max_attempts: 3
+    healthcheck:
+      test: ['CMD-SHELL', 'wget -q --spider http://localhost:8222/healthz || exit 1']
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+
+networks:
+  fluxer-shared:
+    external: true

fluxer/fluxer_devops/nats_core/nats.conf

@@ -0,0 +1,6 @@
+port: 4222
+http_port: 8222
+
+authorization {
+  token: "$NATS_AUTH_TOKEN"
+}