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/20260109193656_admin_api_keys.cql

@@ -0,0 +1,22 @@
+CREATE TABLE IF NOT EXISTS fluxer.admin_api_keys (
+    key_id bigint,
+    key_hash text,
+    name text,
+    created_by_user_id bigint,
+    created_at timestamp,
+    last_used_at timestamp,
+    expires_at timestamp,
+    version int,
+    PRIMARY KEY (key_id)
+);
+
+CREATE TABLE IF NOT EXISTS fluxer.admin_api_keys_by_creator (
+    created_by_user_id bigint,
+    key_id bigint,
+    created_at timestamp,
+    name text,
+    expires_at timestamp,
+    last_used_at timestamp,
+    version int,
+    PRIMARY KEY (created_by_user_id, key_id)
+) WITH CLUSTERING ORDER BY (key_id DESC);