feat(dashboard): add time-frame filtering and used-models-only pricing

- All usage endpoints now accept ?period=today|24h|7d|30d|all|custom
  with optional &from=ISO&to=ISO for custom ranges
- Time-series chart adapts granularity: hourly for today/24h, daily for
  7d/30d/all
- Analytics and Costs pages have period selector buttons with custom
  date-range picker
- Pricing table on Costs page now only shows models that have actually
  been used (GET /models?used_only=true)
- Cache-bust version bumped to v=6

static/css/dashboard.css

@@ -1069,6 +1069,28 @@ body {
     font-size: 0.75rem;
 }
 
+/* Period Selector */
+.period-selector {
+    display: flex;
+    gap: 0.25rem;
+    flex-wrap: wrap;
+}
+
+.period-selector .btn.active {
+    background: var(--blue);
+    color: var(--bg0);
+    border-color: var(--blue);
+}
+
+.input-sm {
+    padding: 0.35rem 0.5rem;
+    font-size: 0.8rem;
+    background: var(--bg1);
+    border: 1px solid var(--bg3);
+    border-radius: 4px;
+    color: var(--fg1);
+}
+
 /* Toast Container */
 .toast-container {
     position: fixed;