feat(ui): contribution graph with daily activity heatmap

Add /v1/activity/daily endpoint returning per-day event counts via
generate_series + LEFT JOIN. Frontend renders an SVG contribution
graph with circles colored by quantile-based thresholds. Clicking a
day navigates to /activity/YYYY-MM-DD showing that day's events.

New /activity/:timespan route parses single dates (YYYY-MM-DD) and
ranges (YYYY-MM-DD..YYYY-MM-DD) from the URL to initialize the
activity timeline filter.

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

crates/moments-api/src/main.rs

@@ -7,11 +7,11 @@ use axum::{
     response::IntoResponse,
     routing::get,
 };
-use chrono::{DateTime, Utc};
+use chrono::{DateTime, NaiveDate, Utc};
 use clap::Parser;
 use moments_core::{EventReader, reshape};
 use moments_data::PgStore;
-use moments_entities::{EventQuery, ProjectSummary, Source, SourceSummary, TimelineItem};
+use moments_entities::{DailyCount, EventQuery, ProjectSummary, Source, SourceSummary, TimelineItem};
 use serde::Deserialize;
 use tower_http::{cors::CorsLayer, trace::TraceLayer};
 use tracing::info;
@@ -56,6 +56,7 @@ async fn main() -> anyhow::Result<()> {
         .route("/v1/events", get(list_events))
         .route("/v1/sources", get(list_sources))
         .route("/v1/projects", get(list_projects))
+        .route("/v1/activity/daily", get(daily_counts))
         .route("/v1/forge/{source}/{*rest}", get(forge_proxy))
         .with_state(state)
         .layer(TraceLayer::new_for_http())
@@ -139,6 +140,22 @@ async fn list_projects(
     Ok(Json(projects))
 }
 
+#[derive(Debug, Deserialize)]
+struct DailyCountsParams {
+    from: Option<NaiveDate>,
+    to: Option<NaiveDate>,
+}
+
+async fn daily_counts(
+    State(state): State<AppState>,
+    Query(params): Query<DailyCountsParams>,
+) -> Result<Json<Vec<DailyCount>>, ApiError> {
+    let to = params.to.unwrap_or_else(|| Utc::now().date_naive());
+    let from = params.from.unwrap_or_else(|| to - chrono::Duration::days(365));
+    let counts = state.store.daily_counts(from, to).await.map_err(internal)?;
+    Ok(Json(counts))
+}
+
 /// Allowlisted forge hosts that the proxy may contact.
 const ALLOWED_HOSTS: &[&str] = &["api.github.com", "git.lair.cafe"];