feat: add monthlyHistory[] per player via getMonthlyHistory + bulk fetch (#6)

Add getMonthlyHistory() to models/player for single-player use and
getAllMonthlyHistoriesFromDB() for bulk fetches (one query, grouped in
memory). Wire monthlyHistory into all player objects returned by
getPlayerDataFromDB and getAllRatingsFromDB. Bulk path pre-fetches in
one query to avoid N extra per-player queries.

public/css/shared.css

@@ -551,3 +551,44 @@ a:hover {
         padding: 8px 6px;
     }
 }
+
+/* ── Delta Pills ──────────────────────────────── */
+
+.delta-pill {
+    display: inline-flex;
+    align-items: center;
+    padding: 2px 8px;
+    border-radius: 999px;
+    font-family: var(--font-mono);
+    font-feature-settings: "tnum", "zero";
+    font-size: 11px;
+    font-weight: 500;
+    margin-top: 4px;
+}
+
+.delta-pill.up {
+    background: var(--up-soft);
+    color: var(--up);
+}
+
+.delta-pill.down {
+    background: var(--down-soft);
+    color: var(--down);
+}
+
+.delta-pill.flat {
+    background: oklch(0.95 0.004 260);
+    color: var(--ink-3);
+}
+
+/* ── Table Header Hints ───────────────────────── */
+
+.th-hint {
+    display: block;
+    font-size: 9.5px;
+    font-weight: 400;
+    text-transform: none;
+    letter-spacing: 0;
+    color: var(--ink-3);
+    margin-top: 2px;
+}