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/20260109233843_relationships_by_target.cql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS fluxer.relationships_by_target (
+    target_user_id bigint,
+    source_user_id bigint,
+    type int,
+    nickname text,
+    since timestamp,
+    version int,
+    PRIMARY KEY (target_user_id, source_user_id, type)
+) WITH CLUSTERING ORDER BY (source_user_id ASC, type ASC);