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>

crates/moments-api/Cargo.toml

@@ -21,3 +21,4 @@ serde_json.workspace = true
 chrono.workspace = true
 clap.workspace = true
 reqwest.workspace = true
+resvg.workspace = true