[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/test-keys/README.md

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+# Test JWT Signing Keys
+
+These ES256 (`prime256v1`) private keys are **test fixtures only** — they are
+mounted into the test SUT container by `docker-compose.test.yml` so the AZ-532
+JWKS / signing tests can exercise a real two-key configuration without any
+runtime setup hooks.
+
+- `kid-test-a.pem` — primary signing key in tests (matches `JwtConfig__ActiveKid`).
+- `kid-test-b.pem` — secondary key kept in JWKS to exercise the rotation overlap
+  acceptance criterion (AZ-532 AC-3).
+
+**Never** copy these into a production secrets directory. Production keys live in
+`secrets/jwt-keys/` and are generated per environment by `scripts/generate-jwt-key.sh`.
+The kid is the filename without `.pem`.

e2e/test-keys/kid-test-a.pem

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+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIKsZoiX7zjchNhuYBaONK+1JCqsVyLbx7chHOn4G1h7toAoGCCqGSM49
+AwEHoUQDQgAEJySg5qLEi+UUiypD6x+41gByjmxfwldVHilU7gpCyIf761sIfC8l
+Dji2yHj8WFqt5k+ZnyXC06kVSKdYpe4qng==
+-----END EC PRIVATE KEY-----

e2e/test-keys/kid-test-b.pem

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+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIFU5IdZXAmhQB2E3LKhtVIN5qIIhh24rmWp2wCmrxlNBoAoGCCqGSM49
+AwEHoUQDQgAETWVNEGK9OHwyVv6qCiatl8e3kIfl8CHtgNEd/6WPgMqzAYOm/C4v
+Lq8klDbUZWdfZkyXf9c6j/H4lFKjSj+jdg==
+-----END EC PRIVATE KEY-----