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/20251229154931_swish_payments.cql

@@ -0,0 +1,28 @@
+CREATE TABLE IF NOT EXISTS fluxer.swish_payments (
+    payment_id text,
+    user_id bigint,
+    product_type text,
+    status text,
+    is_gift boolean,
+    amount int,
+    currency text,
+    message text,
+    payer_alias text,
+    payment_reference text,
+    gift_code text,
+    created_at timestamp,
+    completed_at timestamp,
+    PRIMARY KEY ((payment_id))
+);
+
+CREATE TABLE IF NOT EXISTS fluxer.swish_payments_by_user (
+    user_id bigint,
+    payment_id text,
+    product_type text,
+    status text,
+    is_gift boolean,
+    amount int,
+    currency text,
+    created_at timestamp,
+    PRIMARY KEY ((user_id), created_at)
+) WITH CLUSTERING ORDER BY (created_at DESC);