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/20251206163442_add_admin_archives.cql

@@ -0,0 +1,56 @@
+CREATE TABLE IF NOT EXISTS fluxer.admin_archives_by_subject (
+    subject_type text,
+    subject_id bigint,
+    archive_id bigint,
+    requested_by bigint,
+    requested_at timestamp,
+    started_at timestamp,
+    completed_at timestamp,
+    failed_at timestamp,
+    storage_key text,
+    file_size bigint,
+    progress_percent int,
+    progress_step text,
+    error_message text,
+    download_url_expires_at timestamp,
+    expires_at timestamp,
+    PRIMARY KEY ((subject_type, subject_id), archive_id)
+) WITH CLUSTERING ORDER BY (archive_id DESC);
+
+CREATE TABLE IF NOT EXISTS fluxer.admin_archives_by_requester (
+    requested_by bigint,
+    archive_id bigint,
+    subject_type text,
+    subject_id bigint,
+    requested_at timestamp,
+    started_at timestamp,
+    completed_at timestamp,
+    failed_at timestamp,
+    storage_key text,
+    file_size bigint,
+    progress_percent int,
+    progress_step text,
+    error_message text,
+    download_url_expires_at timestamp,
+    expires_at timestamp,
+    PRIMARY KEY ((requested_by), archive_id)
+) WITH CLUSTERING ORDER BY (archive_id DESC);
+
+CREATE TABLE IF NOT EXISTS fluxer.admin_archives_by_type (
+    subject_type text,
+    archive_id bigint,
+    subject_id bigint,
+    requested_by bigint,
+    requested_at timestamp,
+    started_at timestamp,
+    completed_at timestamp,
+    failed_at timestamp,
+    storage_key text,
+    file_size bigint,
+    progress_percent int,
+    progress_step text,
+    error_message text,
+    download_url_expires_at timestamp,
+    expires_at timestamp,
+    PRIMARY KEY ((subject_type), archive_id)
+) WITH CLUSTERING ORDER BY (archive_id DESC);