Add OAuth2 resource server project: JWT validation, JWKS and key rotation

Companion code for the follow-up article. The repository now holds two Maven
projects sharing one docs/ tree:

  jwt-authentication/       the hand-written filter application (unchanged, moved)
  oauth2-resource-server/   a resource server, a Keycloak compose, and a stub
                            issuer whose JWK Set can be mutated on command

The stub exists because Keycloak will not rotate a signing key at a chosen
second, report how many times its JWKS endpoint was fetched, or drop a key from
the published set on request - and the caching and rotation measurements need
all three. The Keycloak run confirms the same code path against a real issuer.

Findings captured under docs/output/, all from real runs:

  * The default validator stack does not check aud. A token minted for another
    service in the same realm is accepted.
  * Spring Security builds its JWKSource with refreshAheadCache(false) and
    rateLimited(false), overriding two of Nimbus's protective defaults, and
    enables Nimbus caching only when NO Spring cache was supplied - so
    supplying one removes the five-minute expiry.
  * A key retired from the JWK Set stops being accepted at t+300s with the
    default cache, and never with a Spring cache that has no TTL.
  * 25 tokens carrying an unknown kid produce 25 JWKS fetches at the issuer,
    through permitAll() endpoints included.
  * A hyphenated client id in an authorities-claim-expression parses as
    subtraction; the SpelEvaluationException is swallowed and logged at TRACE.
  * A clientScopes key in a Keycloak realm import replaces the built-in scopes
    rather than adding to them.

New docs chapters 12-18. README covers both projects. Existing docs and scripts
updated for the new paths; no docs/output/ file from the first article moved, so
links in the published article still resolve.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013f7f2XZXrQ6gW3RtZE187t

docs/05-hs256-vs-rs256.md

@@ -50,7 +50,7 @@ The decoder side, confusingly, *does* use `macAlgorithm(..)` / `signatureAlgorit
 **The secret must be ≥ 256 bits.** Nimbus enforces the JWA rule that an HMAC key is at
 least as long as its digest; a shorter one throws `KeyLengthException` at encoder
 construction, not at first request.
-[`Hs256KeyConfig`](../src/main/java/com/ankurm/jwtauth/config/Hs256KeyConfig.java) fails
+[`Hs256KeyConfig`](../jwt-authentication/src/main/java/com/ankurm/jwtauth/config/Hs256KeyConfig.java) fails
 fast with a clearer message. A short secret is also brute-forceable offline — the
 attacker has the ciphertext, the plaintext, and unlimited attempts.
 
@@ -84,7 +84,7 @@ key rotation is impossible: the verifier cannot tell which of two published keys
 
 ### Publishing the public half
 
-[`Rs256KeyConfig.JwkSetEndpoint`](../src/main/java/com/ankurm/jwtauth/config/Rs256KeyConfig.java)
+[`Rs256KeyConfig.JwkSetEndpoint`](../jwt-authentication/src/main/java/com/ankurm/jwtauth/config/Rs256KeyConfig.java)
 serves a real JWK Set. From [`rs256-demo.txt`](output/rs256-demo.txt):
 
 ```json