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-client-credentials-opaque.txt

@@ -0,0 +1,68 @@
+
+------------------------------------------------------------------
+== POST /oauth2/token   grant_type=client_credentials
+------------------------------------------------------------------
+$ curl -su demo-service:service-secret -d grant_type=client_credentials \
+       -d scope=orders.read http://localhost:9000/oauth2/token
+{
+    "access_token": "DE0Ps3Jo6f6IdZ1Jd6bqBg_HnYTatIanVFutxdNihyAkHjlgMHWElxo4TGiWI9oNkxgdOAZZu6vgW9BXZrd_KkYVkKrV238fmHUCD9Xpz0_U3k55Brs9fXcMwxxTKGWa",
+    "scope": "orders.read",
+    "token_type": "Bearer",
+    "expires_in": 599
+}
+
+------------------------------------------------------------------
+== Not a JWT
+------------------------------------------------------------------
+The access token is an opaque reference: DE0Ps3Jo6f6IdZ1Jd6bqBg_HnYTatIanVFutxdNihyAkHjlgMHWElxo4TGiWI9oNkxgdOAZZu6vgW9BXZrd_KkYVkKrV238fmHUCD9Xpz0_U3k55Brs9fXcMwxxTKGWa
+Length 128. It carries no claims; the resource server must introspect it.
+
+------------------------------------------------------------------
+== POST /oauth2/introspect
+------------------------------------------------------------------
+{
+    "active": true,
+    "sub": "demo-service",
+    "aud": [
+        "demo-service"
+    ],
+    "nbf": 1787538776,
+    "scope": "orders.read",
+    "iss": "http://localhost:9000",
+    "exp": 1787539376,
+    "iat": 1787538776,
+    "jti": "9d96a819-4f1e-4efb-8816-4523bb6def61",
+    "client_id": "demo-service",
+    "token_type": "Bearer"
+}
+
+------------------------------------------------------------------
+== Wrong secret
+------------------------------------------------------------------
+$ curl -si -u demo-service:WRONG -d grant_type=client_credentials http://localhost:9000/oauth2/token
+HTTP/1.1 401
+{"error":"invalid_client"}
+------------------------------------------------------------------
+== A grant the client is not registered for
+------------------------------------------------------------------
+$ curl -si -u demo-service:service-secret -d grant_type=authorization_code -d code=x http://localhost:9000/oauth2/token
+HTTP/1.1 400
+{"error":"invalid_grant"}
+------------------------------------------------------------------
+== A scope the client is not registered for
+------------------------------------------------------------------
+$ curl -s -u demo-service:service-secret -d grant_type=client_credentials -d scope=orders.write http://localhost:9000/oauth2/token
+{"error":"invalid_scope"}
+
+------------------------------------------------------------------
+== Calling the resource server with the token
+------------------------------------------------------------------
+GET /public -> 401
+  WWW-Authenticate: Bearer error="invalid_token", error_description="An error occurred while attempting to decode the Jwt: Malformed token", error_uri="https://tools.ietf.org/html/rfc6750#section-3.1", resource_metadata="http://localhost:8090/.well-known/oauth-protected-resource"
+
+GET /api/orders -> 401
+  WWW-Authenticate: Bearer error="invalid_token", error_description="An error occurred while attempting to decode the Jwt: Malformed token", error_uri="https://tools.ietf.org/html/rfc6750#section-3.1", resource_metadata="http://localhost:8090/.well-known/oauth-protected-resource"
+
+GET /api/admin -> 401
+  WWW-Authenticate: Bearer error="invalid_token", error_description="An error occurred while attempting to decode the Jwt: Malformed token", error_uri="https://tools.ietf.org/html/rfc6750#section-3.1", resource_metadata="http://localhost:8090/.well-known/oauth-protected-resource"
+