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/20251225170000_add_scheduled_messages.cql

@@ -0,0 +1,14 @@
+CREATE TABLE IF NOT EXISTS fluxer.scheduled_messages (
+    user_id bigint,
+    scheduled_message_id bigint,
+    channel_id bigint,
+    payload text,
+    scheduled_at timestamp,
+    scheduled_local_at text,
+    timezone text,
+    status text,
+    status_reason text,
+    created_at timestamp,
+    invalidated_at timestamp,
+    PRIMARY KEY (user_id, scheduled_message_id)
+) WITH CLUSTERING ORDER BY (scheduled_message_id DESC);