[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>

Azaion.Common/Entities/Session.cs

@@ -0,0 +1,29 @@
+namespace Azaion.Common.Entities;
+
+/// <summary>
+/// AZ-531 — refresh-token session row. One row per issued refresh token. A
+/// "session family" is the chain of rotated sessions that all share the same
+/// <see cref="FamilyId"/>; reuse-detection keys off it.
+/// </summary>
+public class Session
+{
+    public Guid      Id               { get; set; }
+    public Guid      UserId           { get; set; }
+    public string    RefreshHash      { get; set; } = null!;
+    public Guid      FamilyId         { get; set; }
+    public DateTime  IssuedAt         { get; set; }
+    public DateTime  LastUsedAt       { get; set; }
+    public DateTime  ExpiresAt        { get; set; }
+    public DateTime? RevokedAt        { get; set; }
+    public string?   RevokedReason    { get; set; }
+    public Guid?     ParentSessionId  { get; set; }
+    public DateTime  FamilyStartedAt  { get; set; }
+}
+
+public static class SessionRevokedReasons
+{
+    public const string Rotated         = "rotated";
+    public const string ReuseDetected   = "reuse_detected";
+    public const string LoggedOut       = "logged_out";
+    public const string FamilyRevoked   = "family_revoked";
+}