✨(jmap) add JMAP endpoint

This is the first step in our standards-compliant JMAP support. It
implements just enough to be able to list recent threads.

Author: sylvinus

src/backend/core/api/jmap/__init__.py

@@ -0,0 +1 @@
+"""JMAP API module for the messages application."""

src/backend/core/api/jmap/errors.py

@@ -0,0 +1,61 @@
+"""JMAP error types as per RFC 8620."""
+
+
+class JMAPError(Exception):
+    """Base JMAP error."""
+
+    error_type: str = "serverFail"
+
+    def __init__(self, description: str = ""):
+        self.description = description
+        super().__init__(description)
+
+    def to_response(self, call_id: str) -> list:
+        """Convert to JMAP error response tuple."""
+        return [
+            "error",
+            {"type": self.error_type, "description": self.description},
+            call_id,
+        ]
+
+
+class UnknownCapabilityError(JMAPError):
+    """The request used a capability not advertised in the session."""
+
+    error_type = "unknownCapability"
+
+
+class UnknownMethodError(JMAPError):
+    """The method name is not recognized."""
+
+    error_type = "unknownMethod"
+
+
+class InvalidArgumentsError(JMAPError):
+    """One or more arguments are of the wrong type or invalid."""
+
+    error_type = "invalidArguments"
+
+
+class InvalidResultReferenceError(JMAPError):
+    """A result reference could not be resolved."""
+
+    error_type = "invalidResultReference"
+
+
+class AccountNotFoundError(JMAPError):
+    """The accountId does not correspond to a valid account."""
+
+    error_type = "accountNotFound"
+
+
+class ForbiddenError(JMAPError):
+    """The action is forbidden for the authenticated user."""
+
+    error_type = "forbidden"
+
+
+class ServerFailError(JMAPError):
+    """An unexpected server error occurred."""
+
+    error_type = "serverFail"