feat: add web demo for producer/consumer testing with JDK queue

[devin-ai-integration]

Summary

Adds a browser-based demo application (test scope) for interactively testing the itqueue producer/consumer flow using the JDK in-memory queue backend (no Redis required).

New test files:

• DemoApplication.java — Spring Boot entry point excluding Redis and ItQueueAutoConfig
• QueueController.java — REST controller exposing /api/send, /api/send-batch, /api/consumed, /api/produced, /api/errors, /api/clear
• index.html — Single-page UI with producer form, consumer view, live stats, and activity log

pom.xml changes:

• Added spring-boot-starter-web (test scope) for the embedded Tomcat server
• Bumped maven-compiler-plugin from unversioned (3.1) to 3.11.0 and added Lombok annotationProcessorPaths to fix Lombok setter generation during compilation

How to run: mvn test-compile && java -cp "target/classes:target/test-classes:$(mvn dependency:build-classpath -DincludeScope=test -Dmdep.outputFile=/dev/stdout -q)" io.github.ithamal.queue.demo.DemoApplication, then open http://localhost:8080.

Review & Testing Checklist for Human

• maven-compiler-plugin changes affect production build — The version bump to 3.11.0 and the added annotationProcessorPaths are NOT test-scoped; they change how the main artifact is compiled. Verify this doesn't alter the published JAR behavior or break existing CI.
• Unchecked cast in sendBatch — (Integer) body.getOrDefault("count", 5) may throw ClassCastException if Jackson deserializes the JSON number as a different type (e.g., Long). Test with various count values.
• XSS in index.html — ${m.payload} is injected directly into innerHTML without escaping. Low risk since this is test-only, but worth noting if the demo is shared.
• Run the demo locally: send single messages, batch messages, and verify consumed count matches produced count with 0 errors.

Notes

• Message IDs display as null because the JDK queue backend does not assign IDs (only Redis backends do). The consumer.ack(null) call is effectively a no-op in the JDK implementation.
• The in-memory message lists (ConcurrentLinkedQueue) grow unbounded — acceptable for a demo but not production use.

Link to Devin session: https://app.devin.ai/sessions/32eb0f67b07f482ab5e68b341d831103
Requested by: @kenlin8827

[devin-ai-integration]

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