Fluxer PostgreSQL→Cassandra migration tool

Type-aware migration from Fluxer's PostgreSQL fluxer_kv backend to the
native Apache Cassandra schema, plus scylla-feature image builds and a
full cutover runbook.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

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+# Copy to `migrate.env` and fill in. Values are read literally by `docker run --env-file`
+# (no quotes, no shell expansion) — paste passwords with special chars as-is.
+
+# ── Source: PostgreSQL (the existing Fluxer fluxer_kv backend) ────────────────
+PG_HOST=10.1.10.36
+PG_PORT=5432
+PG_USER=fluxer
+PG_PASSWORD=CHANGE_ME
+PG_DATABASE=fluxer
+
+# ── Destination: Apache Cassandra ────────────────────────────────────────────
+SCYLLA_HOSTS=10.1.10.38:9042
+SCYLLA_DC=atl
+CASSANDRA_USERNAME=fluxer
+CASSANDRA_PASSWORD=CHANGE_ME
+
+# ── Tuning (optional) ────────────────────────────────────────────────────────
+# KV_TABLE=fluxer_kv
+# CASSANDRA_KEYSPACE=fluxer
+# BATCH_SIZE=200
+# CONCURRENCY=16