Baseline comparison view #18

Why

A busy hour means little without knowing what a normal week looks like on testnet.

In this repo today

Only the hack window is indexed, and /api/timeline returns deploys per hour for that window (components/charts/timeline-area.tsx draws it in the Overview). There is no baseline data yet, so this issue includes indexing it.

What to build

• A baseline offset in the hackathon YAML and Hackathon model: the same length window a set number of days earlier. sync indexes it into the same tables. Add a test that baseline rows cannot change any project's tier.
• /api/timeline returns baseline counts aligned by hour of the window.
• A toggle on the timeline chart: overlay (the default) or side by side. The Overview stat cards show the change against the baseline as a small secondary line.

Acceptance criteria

• The toggle state persists in the URL search params.
• The tier test above passes.

See CONTRIBUTING.md for conventions.