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/scripts/compile-legacy.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Compiles the SAS 1.x-style configuration against the 7.1.1 classpath and records the
+# compiler's own words. The point is that the error text is what you will actually see,
+# not a paraphrase of it.
+set -u
+cd "$(dirname "$0")/.."
+OUT=../docs/output/as-legacy-compile-failure.txt
+mkdir -p ../docs/output
+
+mvn -B -q -pl auth-server dependency:build-classpath \
+    -Dmdep.outputFile=/tmp/as-cp.txt -DincludeScope=compile >/dev/null 2>&1
+CP=$(cat /tmp/as-cp.txt)
+
+WORK=$(mktemp -d)
+mkdir -p "$WORK/com/ankurm/authserver/legacy" "$WORK/out"
+cp src-broken/LegacySasConfig.java.txt "$WORK/com/ankurm/authserver/legacy/LegacySasConfig.java"
+
+javac -nowarn -d "$WORK/out" -cp "$CP" \
+      "$WORK/com/ankurm/authserver/legacy/LegacySasConfig.java" > "$WORK/err.txt" 2>&1
+STATUS=$?
+
+{
+  echo "# The SAS 1.x configuration, compiled against Spring Boot 4.1.1 / Spring Security 7.1.1."
+  echo "# Source: src-broken/LegacySasConfig.java.txt"
+  echo
+  echo "\$ javac -cp <spring-boot-4.1.1 classpath> LegacySasConfig.java"
+  echo
+  sed "s|$WORK|.|g" "$WORK/err.txt"
+  echo
+  echo "javac exit status: $STATUS"
+} > "$OUT"
+
+rm -rf "$WORK"
+cat "$OUT"