fix: dashboard stale data on browser refresh

[avrabe]

Summary

• Add Cache-Control: no-store to sendHtml() and sendJson() in the dashboard handler
• Prevents browsers from caching HTMX partial responses (deployment panel, summary, repos, PRs, etc.)
• Fixes the issue where refreshing the page shows stale data (deployed SHA, "started X ago", branch count all frozen)

Root cause

The sendHtml and sendJson helpers had no Cache-Control header, so browsers cached XHR responses from HTMX partials. On page reload, the browser served disk-cached HTML instead of hitting the server.

Test plan

• Deploy and verify: refresh the dashboard — deployment SHA, "started ago", and branch count should update on each reload
• Verify HTMX polling still works (partials auto-refresh every 10 min)
• Static assets (htmx.js, idiomorph.js, app.js) still cache normally via their own max-age headers

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