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

oauth2-resource-server/scripts/keycloak-demo.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# The same resource server, the same code, a real Keycloak.
+# Requires: docker compose -f docker/compose.yaml up -d
+#           ./scripts/run-rs.sh keycloak,roles
+set -eu
+. "$(dirname "$0")/lib.sh"
+
+head1 "Keycloak discovery document"
+curl -s "$KC/realms/demo/.well-known/openid-configuration" \
+  | python3 -c 'import json,sys; d=json.load(sys.stdin); [print("  %-22s %s" % (k, d[k])) for k in ("issuer","jwks_uri","token_endpoint")]'
+
+head1 "Keycloak JWK Set"
+curl -s "$KC/realms/demo/protocol/openid-connect/certs" \
+  | python3 -c '
+import json,sys
+d=json.load(sys.stdin)
+print("  keys published:", len(d["keys"]))
+for k in d["keys"]:
+    print("   kid=%s alg=%s use=%s kty=%s" % (k.get("kid"), k.get("alg"), k.get("use"), k.get("kty")))'
+
+head1 "1. alice, password grant"
+T=$(kc_token alice alice-password)
+claims "$T"
+call "GET /api/me" "$RS/api/me" "$T"
+call "GET /api/reports    (client role, from resource_access.reports-api.roles)" "$RS/api/reports" "$T"
+call "GET /api/admin/stats (realm role ADMIN, which alice does not have)" "$RS/api/admin/stats" "$T"
+
+head1 "2. root"
+T=$(kc_token root root-password)
+call "GET /api/admin/stats" "$RS/api/admin/stats" "$T"
+
+head1 "3. nobody - a user with no client role"
+T=$(kc_token nobody nobody-password)
+claims "$T"
+call "GET /api/reports" "$RS/api/reports" "$T"