Add Spring Authorization Server project: OAuth2/OIDC provider, client and resource server

Three modules on Spring Boot 4.1.1 with Spring Authorization Server 7.1.1: the provider
itself, a relying party, and an API that trusts its tokens. Client registration, PKCE,
a custom consent page and token customisation, with profiles that make each failure
reproducible.

Every claim is backed by captured output in docs/output/as-*.txt, regenerated by
authorization-server/scripts/run-all.sh. Notable findings, verified against the jars:

  - OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(HttpSecurity) was deleted
    in 7.0, and both configuration classes moved into spring-security-config
  - ClientSettings.requireProofKey flipped from false to true, on the authorization server
    (1.5.8 -> 7.1.1) and on the OAuth2 client (6.5.1 -> 7.1.1)
  - requireProofKey(false) does not make PKCE optional for a public client; the code
    verifier is that client's only authentication at the token endpoint
  - MediaTypeRequestMatcher(TEXT_HTML) matches Accept: */*, so the token endpoint answers
    API callers with 302 -> /login unless setIgnoredMediaTypes(ALL) is called

Also renames the repository to spring-auth-demo and cross-links the new chapter set from
the existing documentation.

authorization-server/src-broken/LegacySasConfig.java.txt

@@ -0,0 +1,40 @@
+// The configuration every Spring Authorization Server tutorial written before
+// September 2025 tells you to write. It compiled against 1.5.8 and does not compile
+// against 7.1.1. Kept out of the build on purpose - scripts/compile-legacy.sh runs
+// javac against it and captures the real compiler output into
+// docs/output/as-legacy-compile-failure.txt.
+//
+// Three separate breakages in nine lines:
+//   1. the *Configuration class moved out of the SAS jar into spring-security-config
+//   2. the *Configurer class moved AND changed package shape
+//   3. applyDefaultSecurity(HttpSecurity) was deleted outright
+package com.ankurm.authserver.legacy;
+
+import org.springframework.security.oauth2.server.authorization.config.annotation.web.configuration.OAuth2AuthorizationServerConfiguration;
+import org.springframework.security.oauth2.server.authorization.config.annotation.web.configurers.OAuth2AuthorizationServerConfigurer;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
+import org.springframework.security.config.Customizer;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.web.SecurityFilterChain;
+
+@Configuration
+public class LegacySasConfig {
+
+    @Bean
+    @Order(Ordered.HIGHEST_PRECEDENCE)
+    public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http)
+            throws Exception {
+
+        // The 1.x one-liner.
+        OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(http);
+
+        http.getConfigurer(OAuth2AuthorizationServerConfigurer.class)
+                .oidc(Customizer.withDefaults());
+
+        return http.build();
+    }
+}