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/20260114000000_csam_evidence_expirations.cql

@@ -0,0 +1,6 @@
+CREATE TABLE IF NOT EXISTS fluxer.csam_evidence_expirations (
+    bucket text,
+    expires_at timestamp,
+    report_id bigint,
+    PRIMARY KEY ((bucket), expires_at, report_id)
+) WITH CLUSTERING ORDER BY (expires_at ASC);