security(07.3-02): ES256 signing — swap 4 JWT sites + config keys + promote tests

- backend/config.py: add refresh_token_expire_hours=16, jwt_private_key, jwt_public_key
- backend/services/auth.py: add import base64; all 4 JWT sites use ES256 with inline PEM decode; zero HS256/secret_key references
- backend/tests/test_auth_es256.py: promote ES256-01..03 + CFG-01 tests from xfail to passing

backend/tests/test_auth_es256.py

@@ -36,21 +36,47 @@ def es256_keys(monkeypatch):
     monkeypatch.setattr("config.settings.jwt_public_key", pub, raising=False)
 
 
-# ── ES256 algorithm stubs ─────────────────────────────────────────────────────
+# ── ES256 algorithm tests ─────────────────────────────────────────────────────
 
-@pytest.mark.xfail(strict=True, reason="ES256-01: not yet implemented")
 def test_access_token_uses_es256():
-    pytest.xfail("not yet implemented")
+    """ES256-01: access token header must declare alg=ES256."""
+    from services.auth import create_access_token
+    token = create_access_token("u1", "user")
+    # Decode the header (first segment of the JWT)
+    header_b64 = token.split(".")[0]
+    # Add padding to make it valid base64
+    padding = "=" * (4 - len(header_b64) % 4)
+    header = json.loads(base64.urlsafe_b64decode(header_b64 + padding))
+    assert header["alg"] == "ES256"
 
 
-@pytest.mark.xfail(strict=True, reason="ES256-02: not yet implemented")
 def test_hs256_token_rejected():
-    pytest.xfail("not yet implemented")
+    """ES256-02: an HS256 token must raise ValueError when decoded."""
+    import jwt as _jwt
+    from services.auth import decode_access_token
+    hs256_token = _jwt.encode(
+        {
+            "sub": "u1",
+            "typ": "access",
+            "exp": int(time.time()) + 60,
+            "iat": int(time.time()),
+        },
+        "any-hs256-secret",
+        algorithm="HS256",
+    )
+    with pytest.raises(ValueError):
+        decode_access_token(hs256_token)
 
 
-@pytest.mark.xfail(strict=True, reason="ES256-03: not yet implemented")
 def test_reset_token_uses_es256():
-    pytest.xfail("not yet implemented")
+    """ES256-03: password-reset token header must declare alg=ES256 and round-trips."""
+    from services.auth import create_password_reset_token, decode_password_reset_token
+    token = create_password_reset_token("u1")
+    header_b64 = token.split(".")[0]
+    padding = "=" * (4 - len(header_b64) % 4)
+    header = json.loads(base64.urlsafe_b64decode(header_b64 + padding))
+    assert header["alg"] == "ES256"
+    assert decode_password_reset_token(token) == "u1"
 
 
 # ── Startup token rotation stubs ──────────────────────────────────────────────
@@ -87,8 +113,12 @@ async def test_remember_me_cookie_max_age(async_client, auth_user):
     pytest.xfail("not yet implemented")
 
 
-# ── Config field stubs ────────────────────────────────────────────────────────
+# ── Config field tests ────────────────────────────────────────────────────────
 
-@pytest.mark.xfail(strict=True, reason="CFG-01: not yet implemented")
 def test_settings_has_jwt_keys():
-    pytest.xfail("not yet implemented")
+    """CFG-01: Settings must expose jwt_private_key, jwt_public_key, and refresh_token_expire_hours=16."""
+    from config import settings
+    assert hasattr(settings, "jwt_private_key")
+    assert hasattr(settings, "jwt_public_key")
+    assert hasattr(settings, "refresh_token_expire_hours")
+    assert settings.refresh_token_expire_hours == 16