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.

docs/output/as-entrypoint-accept.txt

@@ -0,0 +1,65 @@
+
+------------------------------------------------------------------
+== Public client, failed authentication at the token endpoint   [acceptall profile: setIgnoredMediaTypes NOT called]
+------------------------------------------------------------------
+A public client authenticates at /oauth2/token by presenting a code_verifier.
+With no verifier there is nothing to authenticate with, so the request falls
+through to the AuthenticationEntryPoint - and which entry point runs depends on
+the Accept header.
+
+--- Accept: */*   (curl's default, and most HTTP clients')
+$ curl -H 'Accept: */*' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 302
+Location: http://localhost:9000/login
+
+--- Accept: application/json
+$ curl -H 'Accept: application/json' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 401
+
+--- Accept: text/html   (a browser)
+$ curl -H 'Accept: text/html' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 302
+Location: http://localhost:9000/login
+
+
+------------------------------------------------------------------
+== Confidential client with a wrong secret, for contrast
+------------------------------------------------------------------
+This never reaches the entry point: OAuth2ClientAuthenticationFilter writes the
+error itself, so the Accept header makes no difference.
+HTTP/1.1 401
+
+------------------------------------------------------------------
+== Public client, failed authentication at the token endpoint   [default profile: setIgnoredMediaTypes(ALL) called]
+------------------------------------------------------------------
+A public client authenticates at /oauth2/token by presenting a code_verifier.
+With no verifier there is nothing to authenticate with, so the request falls
+through to the AuthenticationEntryPoint - and which entry point runs depends on
+the Accept header.
+
+--- Accept: */*   (curl's default, and most HTTP clients')
+$ curl -H 'Accept: */*' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 401
+
+--- Accept: application/json
+$ curl -H 'Accept: application/json' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 401
+
+--- Accept: text/html   (a browser)
+$ curl -H 'Accept: text/html' -d grant_type=authorization_code -d code=bogus \
+       -d client_id=demo-spa http://localhost:9000/oauth2/token
+HTTP/1.1 302
+Location: http://localhost:9000/login
+
+
+------------------------------------------------------------------
+== Confidential client with a wrong secret, for contrast
+------------------------------------------------------------------
+This never reaches the entry point: OAuth2ClientAuthenticationFilter writes the
+error itself, so the Accept header makes no difference.
+HTTP/1.1 401