Add custom-validation, etag-caching, restclient-basic-auth: Boot 4.1 API pass

Three companion modules verifying and rewriting the Boot 4.1.1 / Framework 7.0.9 story for three older articles: the javax->jakarta.validation namespace fix plus Jakarta Validation 3.1 record-validation clarification, ETag/ conditional-request APIs re-verified unchanged plus the starter rename, and RestTemplate Basic Auth rebuilt on RestClient with the exchange() trap called out. 19 real passing tests generate every transcript quoted from the three companion articles. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01EQNA6DJ9VgCtW6zhCE8Xud
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+# etag-caching
+
+Companion module for [**Mastering Cache Control with ETag in Spring Boot RESTful APIs**](https://ankurm.com/etag-cache-control-rest-api-spring-boot/)
+on ankurm.com, re-verified against Spring Boot 4.1.1 / Spring Framework 7.0.9.
+
+`mvn test` regenerates every transcript in [`docs/output/`](docs/output).
+
+## Versions
+
+| | |
+|---|---|
+| Spring Boot | 4.1.1 |
+| Spring Framework | 7.0.9 |
+| JDK | Eclipse Temurin 25.0.4.1 (LTS) |
+
+## Quickstart
+
+```bash
+export JAVA_HOME=/path/to/jdk-25
+mvn -DskipTests package
+./scripts/run.sh
+curl -i http://localhost:8080/api/products/42
+mvn test               # 5 tests, regenerates docs/output/
+```
+
+## Endpoints
+
+| Endpoint | Shows |
+|---|---|
+| `GET /api/products/{id}` | deep cache: `WebRequest.checkNotModified()` before the "expensive" part |
+| `PUT /api/products/{id}` | conditional update with `If-Match`, optimistic locking, ETag rotation |
+| `GET /api/echo/{message}` | shallow cache: zero-code `ShallowEtagHeaderFilter` on a scoped URL pattern |
+
+## Documentation
+
+1. [spring-boot-starter-web is deprecated in favour of spring-boot-starter-webmvc](docs/01-starter-web-renamed.md)
+2. [ETags are unchanged, verified rather than assumed](docs/02-etags-unchanged-verified.md)
+3. [Deep cache vs shallow cache, and conditional PUT with If-Match](docs/03-deep-vs-shallow-cache.md)
+
+## Findings worth the trip
+
+- **`spring-boot-starter-web`'s own published POM now says "deprecated in favor of
+  spring-boot-starter-webmvc"** -- read directly off Maven Central, not a migration guide. Both
+  resolve to an identical dependency set today.
+- **`ShallowEtagHeaderFilter`, `WebRequest.checkNotModified()`, and `ResponseEntity.eTag()` are
+  all unchanged** on Spring Framework 7.0.9 -- same packages, same behaviour, confirmed by
+  compiling and running against them rather than assumed from the Boot 3 version of this article.
+- **Registering `ShallowEtagHeaderFilter` as a plain `@Bean` applies it globally**; this module
+  scopes it to one URL pattern with `FilterRegistrationBean` instead, so it does not shadow the
+  deliberately deeper caching on `/api/products`.
+
+## License
+
+MIT -- see [LICENSE](../LICENSE).