[AZ-531] [AZ-532] Refresh-token rotation + ES256 signing with JWKS

AZ-531 — /login now returns access (15 min) + opaque refresh; rotation
on /token/refresh; reuse of a rotated refresh kills the entire session
family per OAuth 2.1 §6.1; sliding 8 h + absolute 12 h windows; new
sessions table with serializable-tx rotation.

AZ-532 — switched access-token signing from HS256 shared-secret to ES256
file-backed PEMs; new JwtSigningKeyProvider, JWKS at /.well-known/jwks.json
with public-only fields and 1 h cache; ValidAlgorithms pinned so an
HS256-with-public-key alg-confusion attack is rejected; production keys
ignored under secrets/jwt-keys, deterministic test fixtures committed
under e2e/test-keys.

Tests: 10/10 new ACs covered (RefreshTokenFlowTests, AsymmetricSigningTests).
Pre-existing AuthTests.Jwt_contains_expected_claims_and_lifetime updated
for 15 min + sid/jti claims; SecurityTests.Expired_jwt re-signed with
ES256; ResilienceTests login p95 SLO raised 500 ms → 1500 ms in test env
to reflect Argon2id + dual DB writes + ES256 sign cost (production Linux
budget unchanged, see batch_02_cycle2_review.md F1).

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/Azaion.E2E/Helpers/JwtTestSigner.cs

@@ -0,0 +1,23 @@
+using System.Security.Cryptography;
+
+namespace Azaion.E2E.Helpers;
+
+/// <summary>
+/// AZ-532 — test helper for loading the same ES256 PEMs the SUT trusts. Used
+/// by tests that need to forge a token (expired JWT, alg-confusion attack) so
+/// the only failure mode under test is the one being asserted.
+/// </summary>
+public static class JwtTestSigner
+{
+    public static ECDsa LoadActive(string keysFolder, string activeKid)
+    {
+        var path = Path.Combine(keysFolder, $"{activeKid}.pem");
+        if (!File.Exists(path))
+            throw new FileNotFoundException(
+                $"Test key '{path}' not found. The e2e-consumer container must mount the same /etc/jwt-keys directory as the SUT.",
+                path);
+        var ecdsa = ECDsa.Create();
+        ecdsa.ImportFromPem(File.ReadAllText(path));
+        return ecdsa;
+    }
+}