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>

Cargo.toml

@@ -30,6 +30,7 @@ anyhow = "1"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "json", "gzip"] }
 figment = { version = "0.10", features = ["toml", "env"] }
 clap = { version = "4", features = ["derive", "env"] }
+resvg = "0.45"
 
 # internal
 moments-entities = { path = "crates/moments-entities", version = "=0.1.0" }