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-entities/src/lib.rs

@@ -84,6 +84,13 @@ pub struct SourceSummary {
     pub latest: Option<DateTime<Utc>>,
 }
 
+/// Per-day event count for the contribution graph.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DailyCount {
+    pub date: chrono::NaiveDate,
+    pub count: i64,
+}
+
 /// Per-repo activity rollup for the dashboard.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ProjectSummary {