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/audience.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# The gap between "the signature is valid" and "this token was meant for me".
+set -u
+cd "$(dirname "$0")/.."
+. scripts/lib.sh
+OUT=../docs/output/as-audience.txt
+mkdir -p ../docs/output
+{
+  section "A token for a DIFFERENT audience, signed by the SAME issuer"
+  echo "demo-service's tokens carry aud=[orders-api] thanks to the token customiser."
+  echo "Here we ask for one and then present it to a resource server configured to"
+  echo "require a different audience - and to one that does not check at all."
+  TOKEN=$(curl -s -u demo-service:service-secret -d grant_type=client_credentials \
+               -d scope=orders.read "$AS/oauth2/token" \
+          | python3 -c 'import sys,json;print(json.load(sys.stdin).get("access_token",""))')
+  echo
+  echo "aud claim in the token:"
+  jwt_payload "$TOKEN" | grep -A3 '"aud"'
+
+  section "Resource server running with demo.validate-audience=false"
+  echo "This is the Spring Boot default: issuer-uri alone validates signature, exp/nbf"
+  echo "and iss. Audience is not checked unless you add a validator."
+  curl -s -o /tmp/b -w 'GET /api/orders -> %{http_code}\n' -H "Authorization: Bearer $TOKEN" "$RS/api/orders"
+  head -c 300 /tmp/b; echo
+
+  section "The same token with a deliberately mangled signature"
+  BAD="${TOKEN%?}X"
+  curl -s -D - -o /dev/null -H "Authorization: Bearer $BAD" "$RS/api/orders" \
+    | sed -n '1p;/^WWW-Authenticate/p'
+
+  section "No token at all"
+  curl -s -D - -o /dev/null "$RS/api/orders" | sed -n '1p;/^WWW-Authenticate/p'
+} > "$OUT" 2>&1
+sed -i 's/[[:space:]]*$//' "$OUT"
+echo "wrote $OUT"