Add the scheduling module

Three replicas of one application against one PostgreSQL database, proving
duplicate @Scheduled execution and then removing it with ShedLock: 24
executions where 8 were due, then 7 for 7 ticks.

Also measured: @SchedulerLock without @EnableSchedulerLock does nothing and
warns about nothing; spring.task.scheduling.pool.size=1 does not starve a
fixedRate job but delays it and fires 35 of 40 executions in a burst; and a
node whose clock is 40 seconds fast takes a live lock unless the provider
uses usingDbTime().

scheduling/docker-compose.yml

@@ -0,0 +1,51 @@
+# Three replicas and one database, which is the smallest deployment in which a @Scheduled method
+# misbehaves. Run `docker compose up --scale app=3` if you would rather set the replica count
+# from the command line; the three named services below exist so that each one gets a stable
+# INSTANCE_ID in the transcript.
+#
+#   docker compose up --build
+#   docker compose exec db psql -U app -d shedlockdemo -c \
+#     "select instance_id, count(*) from job_execution group by instance_id"
+#
+# SPRING_PROFILES_ACTIVE=unlocked reproduces the duplicate executions; switch it to `locked` to
+# watch the count collapse to one per tick.
+
+services:
+  db:
+    image: postgres:17
+    environment:
+      POSTGRES_USER: app
+      POSTGRES_DB: shedlockdemo
+      POSTGRES_HOST_AUTH_METHOD: trust
+    ports: ["5432:5432"]
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U app -d shedlockdemo"]
+      interval: 2s
+      timeout: 3s
+      retries: 20
+
+  replica-1: &replica
+    build: .
+    depends_on:
+      db: { condition: service_healthy }
+    environment:
+      DB_URL: jdbc:postgresql://db:5432/shedlockdemo
+      DB_USER: app
+      INSTANCE_ID: replica-1
+      SPRING_PROFILES_ACTIVE: ${PROFILE:-unlocked}
+
+  replica-2:
+    <<: *replica
+    environment:
+      DB_URL: jdbc:postgresql://db:5432/shedlockdemo
+      DB_USER: app
+      INSTANCE_ID: replica-2
+      SPRING_PROFILES_ACTIVE: ${PROFILE:-unlocked}
+
+  replica-3:
+    <<: *replica
+    environment:
+      DB_URL: jdbc:postgresql://db:5432/shedlockdemo
+      DB_USER: app
+      INSTANCE_ID: replica-3
+      SPRING_PROFILES_ACTIVE: ${PROFILE:-unlocked}