ContextHandler

Handler for deriving encryption context from authenticated user info.

Note: Must be an async function as it may involve I/O operations.

The context handler is called once per request in middleware (after auth), allowing encryption context to be derived from JWT claims, user properties, or other auth-derived data instead of requiring a separate X-Encryption-Context header.

The return value becomes ctx.metadata for subsequent encrypt/decrypt operations and is persisted with encrypted data for later decryption.

Note: ctx.model and ctx.field will be None in context handlers since the handler runs once per request before any specific model/field is known.