Stats: GitHub-style watch-time heatmap

- api/stats gains by_day: local-date -> seconds (same watch-time
  heuristic as by_weekday)
- ?stats renders a Mon..Sun x week grid (30d window, matches the
  page header) with 4 intensity levels + tooltip per cell + legend;
  replaces the by-weekday bar chart the heatmap subsumes
- test_stats covers by_day (sums, slug-scoped, empty case)

server/app.py

@@ -281,6 +281,7 @@ def api_stats(request: Request, days: int = Query(30, ge=1, le=365),
     # viewing; a backward jump >60s within an ep counts as a rewatch
     eps = {}
     watch_sec = [0.0] * 7  # Mon..Sun
+    by_day = {}  # local YYYY-MM-DD -> seconds
     for ts, payload in positions:
         e = json.loads(payload)
         k = (str(e.get("slug_id")), e.get("season_id"), e.get("ep_id"))
@@ -299,9 +300,13 @@ def api_stats(request: Request, days: int = Query(30, ge=1, le=365),
         if e.get("total"):
             s["total"] = e["total"]
         if s["pos"] is not None and 0 < p - s["pos"] <= 90:
-            s["secs"] += p - s["pos"]
-            s["eng" if e.get("lang") == "eng" else "jpn"] += p - s["pos"]
-            watch_sec[datetime.fromtimestamp(ts).weekday()] += p - s["pos"]
+            d = p - s["pos"]
+            s["secs"] += d
+            s["eng" if e.get("lang") == "eng" else "jpn"] += d
+            dt = datetime.fromtimestamp(ts)
+            watch_sec[dt.weekday()] += d
+            k = dt.strftime("%Y-%m-%d")
+            by_day[k] = by_day.get(k, 0) + d
         elif s["pos"] is not None and p < s["pos"] - 60:
             s["rewinds"] += 1
         s["pos"] = p
@@ -362,6 +367,7 @@ def api_stats(request: Request, days: int = Query(30, ge=1, le=365),
         "plays": len(plays),
         "errors": n_errors,
         "by_weekday": [int(x) for x in watch_sec],  # seconds, Mon..Sun
+        "by_day": {k: int(v) for k, v in by_day.items()},  # seconds per local day
         "top_anime": top,
         "top_searches": [q for q, _ in Counter(searches).most_common(5) if q],
         "sub_dub": {"jpn": plays_by_lang.get("jpn", 0), "eng": plays_by_lang.get("eng", 0)},