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/20251206180000_attachment_decay.cql

@@ -0,0 +1,22 @@
+CREATE TABLE IF NOT EXISTS fluxer.attachment_decay_by_id (
+    attachment_id bigint PRIMARY KEY,
+    channel_id bigint,
+    message_id bigint,
+    filename text,
+    size_bytes varint,
+    uploaded_at timestamp,
+    expires_at timestamp,
+    last_accessed_at timestamp,
+    cost double,
+    lifetime_days int,
+    status text
+);
+
+CREATE TABLE IF NOT EXISTS fluxer.attachment_decay_by_expiry (
+    expiry_bucket int,
+    expires_at timestamp,
+    attachment_id bigint,
+    channel_id bigint,
+    message_id bigint,
+    PRIMARY KEY ((expiry_bucket), expires_at, attachment_id)
+);