Requester enrichment: look up Discord users in an external identity source

Part of PAR-427. Not part of v0.

The Discord → Minecraft uuid/username link lives in a hibernia-economy plugin on the live server, not in Keycloak. Rather than coupling taskl.ie to that plugin, add a generic per-workspace identity lookup webhook:

• The workspace sets a URL and a signing secret.
• taskl.ie sends the requester's Discord ID or email and gets back display attributes, for example { "Minecraft username": "...", "uuid": "..." }.
• Results are cached on the requester's attributes and shown on the ticket page.
• hibernia-economy exposes the endpoint, probably through treasury-rest-api. That work is tracked in the Hibernia Economy project.