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/entrypoint-accept.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# Why a failed token request sometimes answers 302 -> /login instead of 401.
+set -u
+cd "$(dirname "$0")/.."
+. scripts/lib.sh
+OUT=../docs/output/as-entrypoint-accept.txt
+LABEL="${1:-}"
+mkdir -p ../docs/output
+probe() {
+  local accept="$1" desc="$2"
+  echo "--- $desc"
+  echo "\$ curl -H 'Accept: $accept' -d grant_type=authorization_code -d code=bogus \\"
+  echo "       -d client_id=demo-spa $AS/oauth2/token"
+  curl -s -i -H "Accept: $accept" -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa "$AS/oauth2/token" \
+    | sed -n '1p;/^[Ll]ocation:/p;/^WWW-Authenticate/p'
+  echo
+}
+{
+  section "Public client, failed authentication at the token endpoint   [$LABEL]"
+  echo "A public client authenticates at /oauth2/token by presenting a code_verifier."
+  echo "With no verifier there is nothing to authenticate with, so the request falls"
+  echo "through to the AuthenticationEntryPoint - and which entry point runs depends on"
+  echo "the Accept header."
+  echo
+  probe "*/*"               "Accept: */*   (curl's default, and most HTTP clients')"
+  probe "application/json"  "Accept: application/json"
+  probe "text/html"         "Accept: text/html   (a browser)"
+
+  section "Confidential client with a wrong secret, for contrast"
+  echo "This never reaches the entry point: OAuth2ClientAuthenticationFilter writes the"
+  echo "error itself, so the Accept header makes no difference."
+  curl -s -i -u demo-web:wrong -d grant_type=client_credentials "$AS/oauth2/token" \
+    | sed -n '1p;/^[Ll]ocation:/p'
+} >> "$OUT" 2>&1
+sed -i 's/[[:space:]]*$//' "$OUT"
+echo "appended $LABEL to $OUT"