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

.gitignore

@@ -16,3 +16,8 @@ spring-batch/scenario-data/
 
 # spring-batch-partitioning: generated shard CSVs and file-based H2 databases
 spring-batch-partitioning/data/
+
+# db-migrations-expand-contract: runtime classpath file written by `mvn package`
+# (maven-dependency-plugin's build-classpath goal) for the standalone MigrationCli
+# and LoadGenerator entry points - machine-specific, regenerated on every build
+db-migrations-expand-contract/cp.txt