[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/Tests/SecurityTests.cs

@@ -113,9 +113,13 @@ public sealed class SecurityTests
     [Fact]
     public async Task Expired_jwt_is_rejected_for_admin_endpoint()
     {
-        // Arrange
-        var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_fixture.JwtSecret));
-        var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256Signature);
+        // Arrange — sign a deliberately-expired token with the same ES256 test key
+        // the SUT trusts. AZ-532 dropped HS256 acceptance, so this test must use the
+        // production signing path or every "is the token expired?" check would short-
+        // circuit on a wrong-algorithm rejection instead.
+        using var ecdsa = JwtTestSigner.LoadActive(_fixture.JwtKeysFolder, _fixture.JwtActiveKid);
+        var key = new ECDsaSecurityKey(ecdsa) { KeyId = _fixture.JwtActiveKid };
+        var creds = new SigningCredentials(key, SecurityAlgorithms.EcdsaSha256);
         var token = new JwtSecurityToken(
             issuer: "AzaionApi",
             audience: "Annotators/OrangePi/Admins",