Redact text and explain the redactions.

Redacts the plain-text request body using the given policy and context, returning the filtered text together with an explanation of every span that was redacted (type, position, and applied filter). Use the c query parameter to select a context and p to select a policy. When output signing is enabled in Admin Settings, the response includes an X-Philter-Signature header containing an ES256 JWT that binds the SHA-256 hash of the response body, the applied policy name and version, a per-response UUID, and an issue timestamp. Verify the signature using the public key from GET /api/signing-key.