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/20260206172322_message_embed_children.cql

@@ -0,0 +1,17 @@
+CREATE TYPE IF NOT EXISTS fluxer.message_embed_child (
+	type text,
+	title text,
+	description text,
+	url text,
+	timestamp timestamp,
+	color int,
+	author frozen<message_embed_author>,
+	provider frozen<message_embed_provider>,
+	thumbnail frozen<message_embed_media>,
+	image frozen<message_embed_media>,
+	video frozen<message_embed_media>,
+	footer frozen<message_embed_footer>,
+	fields frozen<list<message_embed_field>>,
+	nsfw boolean
+);
+ALTER TYPE fluxer.message_embed ADD children frozen<list<message_embed_child>>;