[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/ApiClient.cs

@@ -33,14 +33,27 @@ public sealed class ApiClient : IDisposable
         _httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
     }
 
+    public async Task<LoginResponse> LoginFullAsync(string email, string password, CancellationToken cancellationToken = default)
+    {
+        using var response = await PostAsync("/login", new { email, password }, cancellationToken).ConfigureAwait(false);
+        response.EnsureSuccessStatusCode();
+        var body = await response.Content.ReadFromJsonAsync<LoginResponse>(JsonOptions, cancellationToken)
+            .ConfigureAwait(false);
+        if (body?.AccessToken is not { Length: > 0 })
+            throw new InvalidOperationException("Login response did not contain an access token.");
+        if (body.RefreshToken is not { Length: > 0 })
+            throw new InvalidOperationException("Login response did not contain a refresh token.");
+        return body;
+    }
+
     public async Task<string> LoginAsync(string email, string password, CancellationToken cancellationToken = default)
     {
         using var response = await PostAsync("/login", new { email, password }, cancellationToken).ConfigureAwait(false);
         response.EnsureSuccessStatusCode();
         var body = await response.Content.ReadFromJsonAsync<LoginResponse>(JsonOptions, cancellationToken)
             .ConfigureAwait(false);
-        if (body?.Token is not { Length: > 0 } t)
-            throw new InvalidOperationException("Login response did not contain a token.");
+        if (body?.AccessToken is not { Length: > 0 } t)
+            throw new InvalidOperationException("Login response did not contain an access token.");
         return t;
     }
 
@@ -84,8 +97,11 @@ public sealed class ApiClient : IDisposable
         return _httpClient.PostAsync(url, content, cancellationToken);
     }
 
-    private sealed class LoginResponse
+    public sealed class LoginResponse
     {
-        public string Token { get; init; } = "";
+        public string   AccessToken  { get; init; } = "";
+        public DateTime AccessExp    { get; init; }
+        public string   RefreshToken { get; init; } = "";
+        public DateTime RefreshExp   { get; init; }
     }
 }