Add OAuth2 resource server project: JWT validation, JWKS and key rotation

Companion code for the follow-up article. The repository now holds two Maven
projects sharing one docs/ tree:

  jwt-authentication/       the hand-written filter application (unchanged, moved)
  oauth2-resource-server/   a resource server, a Keycloak compose, and a stub
                            issuer whose JWK Set can be mutated on command

The stub exists because Keycloak will not rotate a signing key at a chosen
second, report how many times its JWKS endpoint was fetched, or drop a key from
the published set on request - and the caching and rotation measurements need
all three. The Keycloak run confirms the same code path against a real issuer.

Findings captured under docs/output/, all from real runs:

  * The default validator stack does not check aud. A token minted for another
    service in the same realm is accepted.
  * Spring Security builds its JWKSource with refreshAheadCache(false) and
    rateLimited(false), overriding two of Nimbus's protective defaults, and
    enables Nimbus caching only when NO Spring cache was supplied - so
    supplying one removes the five-minute expiry.
  * A key retired from the JWK Set stops being accepted at t+300s with the
    default cache, and never with a Spring cache that has no TTL.
  * 25 tokens carrying an unknown kid produce 25 JWKS fetches at the issuer,
    through permitAll() endpoints included.
  * A hyphenated client id in an authorities-claim-expression parses as
    subtraction; the SpelEvaluationException is swallowed and logged at TRACE.
  * A clientScopes key in a Keycloak realm import replaces the built-in scopes
    rather than adding to them.

New docs chapters 12-18. README covers both projects. Existing docs and scripts
updated for the new paths; no docs/output/ file from the first article moved, so
links in the published article still resolve.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013f7f2XZXrQ6gW3RtZE187t

oauth2-resource-server/src/main/java/com/ankurm/stubissuer/StubSecurityConfig.java

@@ -0,0 +1,21 @@
+package com.ankurm.stubissuer;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.web.SecurityFilterChain;
+
+/**
+ * The stub issuer authenticates nobody. Defining any {@code SecurityFilterChain} bean makes
+ * Boot’s default chain back off, which is the whole purpose of this class.
+ */
+@Configuration
+public class StubSecurityConfig {
+
+	@Bean
+	SecurityFilterChain open(HttpSecurity http) throws Exception {
+		return http.csrf((csrf) -> csrf.disable())
+			.authorizeHttpRequests((auth) -> auth.anyRequest().permitAll())
+			.build();
+	}
+}