[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/appsettings.test.json

@@ -4,6 +4,7 @@
   "AdminPassword": "Admin1234",
   "UploaderEmail": "uploader@azaion.com",
   "UploaderPassword": "Upload1234",
-  "JwtSecret": "TestSecretKeyThatIsAtLeast32CharactersLong123!",
+  "JwtKeysFolder": "/etc/jwt-keys",
+  "JwtActiveKid": "kid-test-a",
   "TestDbConnectionString": "Host=test-db;Port=5432;Database=azaion;Username=postgres;Password=test_password"
 }