feat(api): server-rendered OG image of all-time contribution graph

Add /v1/og/contributions.png endpoint that builds an SVG of the
all-time weekly contribution graph (one row per year) from daily
counts, then rasterizes to PNG via resvg. Served with 1h cache.

Add og:image and twitter:card meta tags to index.html pointing at
the endpoint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/index.html

@@ -4,6 +4,12 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>rob.tn</title>
+    <meta property="og:title" content="rob thijssen" />
+    <meta property="og:description" content="contribution history across github, gitea, and mozilla hg" />
+    <meta property="og:image" content="https://rob.tn/api/v1/og/contributions.png" />
+    <meta property="og:type" content="website" />
+    <meta name="twitter:card" content="summary_large_image" />
+    <meta name="twitter:image" content="https://rob.tn/api/v1/og/contributions.png" />
     <link rel="icon" type="image/x-icon" href="/favicon.ico" />
     <link rel="icon" type="image/png" sizes="16x16" href="/favicon-16.png" />
     <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32.png" />