feature: enable virtual threads, migrate RestTemplate to RestClient, remove stray root properties file - #70

Summary

Runtime-configuration slice on top of the Java 21 / Spring Boot 3.5 upgrade (#69). Three things, all verified against a running app rather than a compile:

1. Virtual threads actually on. The repo had no classpath application.properties at all — the one at the repo root is not under src/main/resources, so Spring never read it. Added src/main/resources/application.properties with spring.threads.virtual.enabled=true, and deleted the dead root file (its only content was a commented-out #server.port = 8081, carried over into the real file). The dead file also polluted the /topic/file/operation listing, which walks the process working directory; that listing is now one entry shorter.

Proven with a throwaway /thread-probe endpoint (removed before commit):

VirtualThread[#44,tomcat-handler-0]/runnable@ForkJoinPool-1-worker-1 virtual=true

2. RestTemplate → RestClient. RestTemplate is in maintenance mode; RestClient is the modern synchronous client. RestTemplateConfig → RestClientConfig (now building from the auto-configured RestClient.Builder), and the quote fetch becomes:

Quote quote = restClient.get().uri(QUOTE_URL).retrieve().body(Quote.class);

The important part is that startup resilience does not regress — the upstream quote service gturnquist-quoters.cfapps.io is dead, and the app must log a WARN and keep going (the bug the base PR just fixed). Verified empirically rather than assumed: RestClient wraps the connection failure in ResourceAccessException, which is a RestClientException, so the existing catch clause still applies unchanged. Running the built jar:

INFO  hello.Application : Started Application in 1.129 seconds
WARN  hello.Application : Could not fetch a quote from http://gturnquist-quoters.cfapps.io/api/random: I/O error on GET request ...

3. Stream cleanup in Application.run(...): Arrays.asList(...).stream().map(...).collect(Collectors.toList()) → Stream.of(...).<Object[]>map(...).toList(). Safe because the result is only iterated and handed to batchUpdate, never mutated.

Verification

• ./mvnw clean verify passes on JDK 21.
• App run via ./mvnw spring-boot:run and the built jar; every endpoint curled. JSON shapes unchanged: / → {"id":1,"content":"Hello, World!"}, /topic → [{"id":...,"subjectName":...,"subjectDescription":...}]. /topic GET/POST/PUT/DELETE, /topic/sort, /topic/minimum/length/{n}, /topic/string/operation, /topic/file/operation, /datetime all behave as before.
• Pre-existing and unrelated: GET /topic/1 (a non-existent id) returns 500 on the base branch too — TopicService calls Optional.get() on a missing topic. Not fixed here; service/ is outside this slice.

No files outside this slice were modified; pom.xml untouched.

Link to Devin session: https://app.devin.ai/sessions/57144a6d7b48468bbe7d6e587aabf483

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