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/rs-startup-failure.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# What happens when the resource server starts and the issuer is not reachable.
+# Worth capturing because the failure is at STARTUP, not at first request - which means
+# a provider outage during a rolling deploy takes your API down with it.
+set -u
+cd "$(dirname "$0")/.."
+. scripts/lib.sh
+OUT=../docs/output/as-rs-startup-failure.txt
+mkdir -p ../docs/output
+kill_app ResourceServerApplication 8090
+kill_app AuthServerApplication 9000
+sleep 1
+mvn -B -o -pl resource-server org.springframework.boot:spring-boot-maven-plugin:run \
+    > /tmp/rs-fail.log 2>&1 || true
+{
+  echo "# resource-server started with spring.security.oauth2.resourceserver.jwt.issuer-uri"
+  echo "# pointing at an authorization server that is not running."
+  echo
+  grep -E '^Caused by|Unable to resolve the Configuration' /tmp/rs-fail.log \
+    | sed 's/^Caused by: //' | head -8
+} > "$OUT"
+cat "$OUT"