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/20260207160233_user_connections.cql

@@ -0,0 +1,16 @@
+CREATE TABLE IF NOT EXISTS fluxer.user_connections (
+    user_id bigint,
+    connection_type text,
+    connection_id text,
+    identifier text,
+    name text,
+    verified boolean,
+    visibility_flags int,
+    sort_order int,
+    verification_token text,
+    verified_at timestamp,
+    last_verified_at timestamp,
+    created_at timestamp,
+    version int,
+    PRIMARY KEY ((user_id), connection_type, connection_id)
+) WITH CLUSTERING ORDER BY (connection_type ASC, connection_id DESC);