U2: Define core Convex domain model

[breadback00-dev]

What is the work

Create the backend domain model for the carousel workflow: formats, projects, source items, slide instances, assets, and intake runs.

Planned areas:

• convex/schema.ts
• convex/formats.ts
• convex/projects.ts
• convex/sourceItems.ts
• convex/slides.ts
• convex/assets.ts
• convex/intake.ts
• convex/*.test.ts

Why does it matter

The Format Library, agent intake lane, visual studio, and SVG export path all need shared durable concepts. If this layer is vague or blob-like, later work will drift and the app will be hard to test.

What does done mean

• Convex schema and functions can represent an Events Roundup project.
• Intake results are separate from editor-approved source items.
• Slide instances can be generated from approved project content later.
• Uploaded image assets and source image URLs can both be represented.
• Backend tests cover the happy path, edge cases, and invalid references.

Acceptance criteria

• Create an Events Roundup project from a seeded or placeholder format.
• Add structured source items to a project and query them in order.
• Allow missing optional item details while marking them reviewable.
• Reject invalid required project or format references.
• Represent uploaded-file assets and external image URL assets distinctly.

Blockers

• Blocked by U1: Scaffold Next.js, Convex, and test harness #1.

What can make this fail

• Domain state is stored as undifferentiated JSON blobs that later UI/tests cannot reason about.
• Intake output and approved editor state are conflated.
• Asset modeling assumes only uploads or only URLs, breaking the chosen MVP image workflow.
• Tests assert implementation internals rather than workflow behavior.

How should Codex verify it

• Run Convex/backend tests for all new domain functions.
• Add at least one workflow-shaped test from project creation through source item insertion.
• Confirm the schema supports the later issue requirements without overbuilding a format builder.
• Check that generated Convex files and secrets are handled correctly.