docs: drop the replica-lag testing runbook from the open-source tree

It describes our local two-server MySQL replication setup, which is
internal tooling rather than anything a self-hoster or contributor
needs. It now lives in the heyputer repo alongside the other internal
docs.

The suite it documented stays: the header comment now states the
env-var gate directly instead of pointing at a file this repo no
longer carries.

doc/testing-replica-lag.md

@@ -1,107 +0,0 @@
-# Testing against a lagging read replica
-
-Some bugs only exist between a primary and a replica that has not caught up:
-a row is deleted on the primary, a reader is served the old copy by the
-replica, and writes referencing it then fail their foreign keys.
-
-Local development runs on sqlite, which has neither a replica nor foreign-key
-enforcement, so neither half of that can happen. Unit tests inject the driver
-error instead. This setup runs the real thing: two MySQL servers in actual
-replication, where `STOP REPLICA` freezes the follower on demand.
-
-## Setup
-
-Assumes the usual local `puter-mysql` container (mysql:8, port 3306,
-`root`/`puter`). Start a follower on 3307:
-
-```bash
-docker run -d --name puter-mysql-replica \
-  -e MYSQL_ROOT_PASSWORD=puter -e MYSQL_DATABASE=puter \
-  -p 3307:3306 mysql:8 \
-  --server-id=2 --log-bin=mysql-bin --relay-log=relay-bin
-
-until docker exec puter-mysql-replica mysqladmin -uroot -pputer ping >/dev/null 2>&1
-do sleep 2; done
-```
-
-Give the primary a replication user:
-
-```bash
-docker exec puter-mysql mysql -uroot -pputer -e "
-CREATE USER IF NOT EXISTS 'repl'@'%' IDENTIFIED BY 'replpass';
-GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%';
-FLUSH PRIVILEGES;"
-```
-
-Seed the follower and point it at the primary. `--source-data=2` records the
-binlog coordinates the dump was taken at, which is where the follower starts:
-
-```bash
-docker exec puter-mysql mysqldump -uroot -pputer \
-  --source-data=2 --single-transaction --databases puter > /tmp/primary.sql
-
-grep -m1 'CHANGE REPLICATION SOURCE' /tmp/primary.sql
-# -- CHANGE REPLICATION SOURCE TO SOURCE_LOG_FILE='binlog.000006', SOURCE_LOG_POS=717382;
-
-docker exec -i puter-mysql-replica mysql -uroot -pputer < /tmp/primary.sql
-
-PRIMARY_IP=$(docker inspect puter-mysql \
-  --format '{{.NetworkSettings.Networks.bridge.IPAddress}}')
-
-docker exec puter-mysql-replica mysql -uroot -pputer -e "
-STOP REPLICA; RESET REPLICA ALL;
-CHANGE REPLICATION SOURCE TO
-  SOURCE_HOST='${PRIMARY_IP}', SOURCE_PORT=3306,
-  SOURCE_USER='repl', SOURCE_PASSWORD='replpass',
-  SOURCE_LOG_FILE='<file from above>', SOURCE_LOG_POS=<pos from above>,
-  GET_SOURCE_PUBLIC_KEY=1;
-START REPLICA;"
-```
-
-`GET_SOURCE_PUBLIC_KEY=1` is required because MySQL 8 defaults to
-`caching_sha2_password` and this link has no TLS. Confirm both threads are up:
-
-```bash
-docker exec puter-mysql-replica mysql -uroot -pputer -e "SHOW REPLICA STATUS\G" \
-  | grep -E 'Replica_IO_Running|Replica_SQL_Running:|Last_Error'
-```
-
-## Running
-
-```bash
-PUTER_TEST_REPLICA_LAG=1 npx vitest run \
-  --config src/backend/vitest.config.ts \
-  src/backend/stores/replicaLag.integration.test.ts
-```
-
-Without the env var the suite skips, so it stays inert in CI and for anyone
-without the containers. It builds its own throwaway database
-(`puter_replica_lag_verify`), runs the MySQL migrations into it, and drops it
-afterwards — your dev data is never touched.
-
-Container names are overridable via `PUTER_TEST_REPLICA_PRIMARY` and
-`PUTER_TEST_REPLICA_FOLLOWER`.
-
-## Writing a case
-
-`freeze()` stops replication; everything after it exists only on the primary.
-An `afterEach` thaws unconditionally, so a failing assertion cannot strand the
-follower and starve later cases of their fixture rows.
-
-```ts
-const app = await makeApp(server, user.id);
-await settle(); // let the follower catch up
-await server.stores.app.getByUid(app.uid); // warm the cache
-
-freeze(); // follower is now behind
-await server.stores.app.delete(app.id); // primary only
-
-expect(await server.stores.app.getByUid(app.uid)).toBeNull();
-```
-
-## Teardown
-
-```bash
-docker rm -f puter-mysql-replica
-docker exec puter-mysql mysql -uroot -pputer -e "DROP USER IF EXISTS 'repl'@'%';"
-```

src/backend/stores/replicaLag.integration.test.ts

@@ -25,8 +25,8 @@
  * unit tests inject them, since sqlite has neither a replica nor FK
  * enforcement.
  *
- * Needs both containers, so it is opt-in and skipped in CI — setup and usage in
- * doc/testing-replica-lag.md.
+ * Needs both containers, so it is opt-in behind `PUTER_TEST_REPLICA_LAG` and
+ * skipped everywhere it is unset, CI included.
  */
 
 import { execFileSync } from 'node:child_process';