Pairing opportunity: add a fixture-backed instant demo path

[eshaanag]

Goal

Add a small fixture-backed demo path so CommitIQ can show a useful repository health dashboard without requiring a live GitHub ingestion run.

This is intended as a paired contribution: one person can own backend fixture loading and the other can own frontend demo entry/display.

Why this matters

The live product is easier to understand when a visitor can inspect a realistic dashboard immediately. This also helps with README demos, screenshots, and Starstruck outreach.

Suggested scope

• Add a backend fixture dataset with representative commit, file, hotspot, and ownership-risk data.
• Add an API route or mode that returns the fixture data without cloning a repository.
• Add a frontend entry point or demo button that loads the fixture-backed view.
• Make the path clearly labeled as demo data.
• Add tests for the backend fixture route and the frontend demo trigger.

Acceptance criteria

• A fresh local setup can open the frontend and load a useful demo state without external API keys.
• Demo mode does not interfere with real repository ingestion.
• CI remains green.
• The PR includes both contributors via commit trailer, for example:

Co-authored-by: Name <email@example.com>

Notes

This issue is intentionally scoped for a co-authored PR so the implementation can demonstrate collaboration rather than just badge chasing.