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/20251217020455_empty_buckets.cql

@@ -0,0 +1,6 @@
+CREATE TABLE IF NOT EXISTS fluxer.channel_empty_buckets (
+    channel_id bigint,
+    bucket int,
+    updated_at timestamp,
+    PRIMARY KEY ((channel_id), bucket)
+) WITH CLUSTERING ORDER BY (bucket DESC);