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/20251225000000_expression_packs.cql

@@ -0,0 +1,18 @@
+CREATE TABLE IF NOT EXISTS fluxer.expression_packs (
+    pack_id bigint PRIMARY KEY,
+    pack_type text,
+    creator_id bigint,
+    name text,
+    description text,
+    created_at timestamp,
+    updated_at timestamp,
+    version int
+);
+
+CREATE TABLE IF NOT EXISTS fluxer.pack_installations (
+    user_id bigint,
+    pack_id bigint,
+    pack_type text,
+    installed_at timestamp,
+    PRIMARY KEY (user_id, pack_id)
+);