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/authorization-server/09-entry-point.md

@@ -0,0 +1,71 @@
+[← 08 The relying party](08-client.md) · [index](README.md) · next: [10 — Should you run one at all](10-should-you.md)
+
+# The entry point and the Accept header
+
+## The symptom
+
+A failed token request answers `302 -> /login` instead of a JSON `401`. Your API client
+follows the redirect, gets 200 and an HTML login page, and reports “the token endpoint
+returned HTML”.
+
+## The cause
+
+The authorization server chain needs two behaviours from one entry point: send a *browser*
+hitting `/oauth2/authorize` to the login page, and send a *machine* hitting `/oauth2/token`
+a protocol error. The documented way to express that is:
+
+```java
+.exceptionHandling(ex -> ex.defaultAuthenticationEntryPointFor(
+        new LoginUrlAuthenticationEntryPoint("/login"),
+        new MediaTypeRequestMatcher(MediaType.TEXT_HTML)))
+```
+
+On its own, that does not work. `MediaTypeRequestMatcher` treats `*/*` as matching
+`text/html`, and `*/*` is what curl, most HTTP clients, and anything that does not set
+`Accept` send. So the matcher fires for API callers too.
+
+## The fix
+
+```java
+MediaTypeRequestMatcher matcher = new MediaTypeRequestMatcher(MediaType.TEXT_HTML);
+matcher.setIgnoredMediaTypes(Set.of(MediaType.ALL));
+```
+
+## The difference, measured
+
+[`as-entrypoint-accept.txt`](../output/as-entrypoint-accept.txt), same request three ways
+against both configurations:
+
+| `Accept` | without `setIgnoredMediaTypes` | with it |
+|---|---|---|
+| `*/*` | **302 → /login** | **401** |
+| `application/json` | 401 | 401 |
+| `text/html` | 302 → /login | 302 → /login |
+
+The browser case is preserved either way. Only the `*/*` case changes, and that is the case
+every API client falls into.
+
+## Why only public clients hit it
+
+A confidential client presenting a wrong secret never reaches the entry point at all:
+`OAuth2ClientAuthenticationFilter` writes the error itself, so the `Accept` header makes no
+difference and you get a clean 401. It is the *public* client — whose only
+authentication mechanism is the code verifier — that falls through to the entry point
+when there is nothing to authenticate with. Which means the bug is invisible until you add
+your first SPA.
+
+## The mirror image in the test suite
+
+```java
+this.mvc.perform(post("/oauth2/token")
+                .accept(MediaType.ALL)
+                .param("grant_type", "authorization_code")
+                .param("code", "bogus")
+                .param("client_id", "demo-spa"))
+        .andExpect(status().isUnauthorized());
+```
+
+Pinning it as a test matters because the fix is one line in an `exceptionHandling` lambda
+and is exactly the kind of thing a later refactor drops.
+
+Next: [10 — Should you run one at all](10-should-you.md)