Bootstrap Convex backend and replace seeded DemoData

[LaloLalo1999]

Tracks Linear AGE-48.

Goal

Replace in-memory demoData with a persistent Convex backend.

Acceptance

• convex dep installed; project initialized via npx convex dev (user picks account/org/project)
• Convex deployment URL captured in .env.local
• Schema covers all DemoData entities (Region, Job, Spec, Approval, Agent, Run, Event, Artifact, Memory, Skill, Marketplace, Trust, Risk, Cost, RepoScan, BacklogIssue)
• Existing immutable helpers ported to Convex mutations
• npm run lint, typecheck, test, build green

Linear: https://linear.app/agentic-eng/issue/AGE-48

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