Add repo names to heatmap tooltip

Extends the JSON response to include per-day repo names via
array_agg on action.repo_name, and surfaces them in the hover
tooltip below the contribution count.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -12,9 +12,9 @@ JSON, and let a custom profile template render the squares client-side.
 ## What it shows / what it doesn't
 
 - ✅ Daily counts (the green squares)
-- ✅ Hover tooltips with the date and count
-- ❌ Repo names, commit messages, branches, file content — none of that ever
-  leaves the database. Only counts.
+- ✅ Hover tooltips with the date, count, and repo names
+- ❌ Commit messages, branches, file content — none of that ever leaves the
+  database.
 
 ## Architecture
 
@@ -102,7 +102,7 @@ Set `OP_TYPES=5,6,7,9,11,18,24` for a more inclusive count.
 
 ## Endpoints
 
-- `GET /heatmap/{username}.json` — JSON `[{"date":"YYYY-MM-DD","count":N}, ...]`
+- `GET /heatmap/{username}.json` — JSON `[{"date":"YYYY-MM-DD","count":N,"repos":["name", ...]}, ...]`
   for the past ~53 weeks. 1-hour cache header.
 - `GET /healthz` — 200 if the DB is reachable, 503 otherwise.