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/15-jwks-caching-and-rotation.md

@@ -178,3 +178,11 @@ JWK Set its own dedicated cache; do not point it at a cache you share with anyth
 ---
 
 [← the authentication converter](14-authentication-converter.md) · [next: what an unknown kid costs →](16-jwks-amplification.md)
+
+---
+
+The provider side of rotation — generating, publishing and retiring the keys this
+chapter watches from the outside — is
+[`authorization-server/02`](authorization-server/02-minimum-provider.md), and why a demo
+provider regenerating its keypair per boot is a feature rather than a bug is in
+[`authorization-server/10`](authorization-server/10-should-you.md).