Add db-migrations-expand-contract: zero-downtime schema migrations proven with a real 4-deploy rolling run

Companion code for Zero-Downtime Database Migrations: Expand-Contract in Practice
with Spring Boot: a full expand/migrate-writes/migrate-reads/contract sequence run
as an actual rolling deploy across two live replicas, with a load generator sending
continuous HTTP traffic through all four deploys (99.98% success, every residual
error traced to a root cause rather than left unexplained). Findings include a real
NOT NULL constraint trap in the expand migration, a backfill-window bug in the read
switch, H2's AUTO_SERVER=TRUE single-point-of-failure behavior under a rolling
restart, the drain-before-SIGTERM fix needed to close a health-check gap during
graceful shutdown, and H2 silently discarding a concurrently committed INSERT during
an ALTER TABLE ADD/DROP COLUMN rebuild - confirmed, by primary source, to be an
H2-specific behavior rather than a property of the technique itself.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019Fb7vW8vLyLKngBc4R3huA

db-migrations-expand-contract/scripts/start-db-server.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Starts H2 as its own standalone TCP server process - not owned by, or co-located
+# inside, either app replica. This is what a real production database is: a process
+# that outlives every app deploy. See docs/12-the-auto-server-trap.md for what went
+# wrong the first time this demo shared a database file directly between the two
+# replicas instead.
+set -euo pipefail
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$DIR/env.sh"
+
+nohup java -cp "$(cat "$CP_FILE")" org.h2.tools.Server \
+    -tcp -tcpPort "$EC_DB_TCP_PORT" -baseDir "$EC_DB_BASE_DIR" -ifNotExists \
+    > "$EC_LOG_DIR/db-server.log" 2>&1 < /dev/null &
+echo $! > "$EC_PID_DIR/db-server.pid"
+wait_db_server
+echo "H2 TCP server up on port $EC_DB_TCP_PORT (pid $(cat "$EC_PID_DIR/db-server.pid")), baseDir $EC_DB_BASE_DIR"