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/cassandra/migrations/20251202000000_push_devices.cql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS fluxer.push_devices (
+    user_id bigint,
+    device_id text,
+    fcm_token text,
+    platform text,
+    device_name text,
+    created_at timestamp,
+    updated_at timestamp,
+    PRIMARY KEY (user_id, device_id)
+);