Add a read-only administration UI for the model

Problem/Motivation

Before anyone edits a structure they need to see what exists: which bundles have endpoints, what each returns, which fields are unmapped, which conventions are unfilled. That view is also the answer to the most common support question a module like this gets — why is this key not in my response.

Proposed resolution

• A listing of endpoints with their kind, path, bundle and structure.
• A structure page showing every property with its source, shape and settings, marking inherited ones and unmapped fields.
• A conventions page showing every role and its state — filled, suggested, empty — read-only at this stage; the buttons arrive with the change queue.
• Everything behind the module's administration permission, and nothing on it that writes.

Remaining tasks

Everything.