Add per-envelope JWT authentication and validation

Introduced a new `EnvelopeReceiverJwt` authentication scheme to support per-envelope JWT validation using cookies specific to envelope keys. Added the `CheckEnvelopeReceiver` endpoint in `AuthController.cs` to validate these tokens, protected by the `AuthPolicy.Receiver` policy.

Configured the `EnvelopeReceiverJwt` scheme to dynamically resolve issuer signing keys and validate tokens. Enhanced `JwtBearerEvents.OnMessageReceived` to extract envelope keys from the request path and retrieve tokens from corresponding cookies.

Updated the `AuthPolicy.Receiver` policy to use the `EnvelopeReceiverJwt` scheme, ensuring isolated authentication for per-envelope scenarios. Added XML documentation for the `CheckEnvelopeReceiver` method.

EnvelopeGenerator.API/Controllers/AuthController.cs

@@ -73,4 +73,17 @@ public partial class AuthController(IOptions<AuthTokenKeys> authTokenKeyOptions,
         => role is not null && !User.IsInRole(role)
             ? Unauthorized()
             : Ok();
+
+    /// <summary>
+    /// Checks whether the caller holds a valid per-envelope receiver token for the given envelope key.
+    /// The request must carry a cookie named <c>AuthTokenSignFLOWReceiver.{envelopeKey}</c>.
+    /// </summary>
+    /// <param name="envelopeKey">The unique envelope key extracted from the route.</param>
+    /// <response code="200">Valid per-envelope token found.</response>
+    /// <response code="401">Token is missing, expired or invalid.</response>
+    [ProducesResponseType(typeof(void), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(void), StatusCodes.Status401Unauthorized)]
+    [Authorize(Policy = AuthPolicy.Receiver)]
+    [HttpGet("check/envelope/{envelopeKey}")]
+    public IActionResult CheckEnvelopeReceiver([FromRoute] string envelopeKey) => Ok();
 }