[codex] Harden dynamic media preset graph node

[gateway]

Summary

This PR hardens Graph Studio's Media Preset path around one schema-driven Media Preset node instead of generated per-preset nodes.

It adds a backend preset catalog for node definitions, dynamic preset fields and image-slot ports, runtime validation for selected models and required slots, and browser-facing helpers so selecting a preset updates the visible fields/model options without stale generated node definitions.

It also reduces graph history noise by adding lightweight run summary endpoints and caps history loading to the recent run summaries rather than pulling full run payloads by default.

Validation

• ./scripts/with_shared_python.sh -m pytest apps/api/tests/test_graph_studio.py -q
• npm --workspace apps/web run test -- graph-media-preset graph-node-field graph-tabs graph-media-preview
• npm --workspace apps/web run test -- studio-gallery media-studio-helpers studio-composer-restore studio-attachment-staging studio-polling studio-shortcuts media-payload
• npm run typecheck:web
• npm run lint:web
• npm run build:web
• git diff --check
• Browser verified Dream Magazine Cover Portrait Demo in Graph Studio: reference image -> dynamic Media Preset -> preview completed with generated asset asset_4af76e9f143c and no visible runtime error.

Notes

• No development docs, task lists, local data, output files, or secrets are included in the branch diff.
• This keeps old prompt recipe normalization behavior, but removes the stale per-preset graph node generation path.