doc: add TEA diagrams and graph schema

[MChorfa]

Summary

This PR is a focused split from #235 and contains only visual and model-supporting artifacts:

• doc/tea_consumer_flow.excalidraw
• doc/tea_data_model.excalidraw
• doc/tea_discovery_security.excalidraw
• doc/tea_publisher_flow.excalidraw
• graph-schema.json

Scope

• no protobuf or JSON schema files
• no implementation artifacts
• no narrative auth, requirements, or versioning text

Intent

Keep review on the diagrams and graph/data-model support material only.

[MChorfa]

This PR is one part of the split from #235, which has been closed in favor of smaller review units.

Related PRs:

• doc: narrow TEA authentication guidance #236 doc: narrow TEA authentication guidance
• spec: add TEA discovery and consumer contract drafts #237 spec: add TEA discovery and consumer contract drafts
• doc: add a short TEA profiles note #238 doc: add a short TEA profiles note
• spec: add TEA publisher and insights service drafts #239 spec: add TEA publisher and insights service drafts
• doc: add TEA narrative drafts #240 doc: add TEA narrative drafts
• doc: add TEA diagrams and graph schema #241 doc: add TEA diagrams and graph schema

[oej]

We already have mermaid diagrams. Please update those if needed instead of adding a new set of diagrams in a new format. Thank you.