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

src/dashboard/models.rs

@@ -33,6 +33,8 @@ pub(super) struct ModelListParams {
     pub reasoning: Option<bool>,
     /// Only models that have pricing data.
     pub has_cost: Option<bool>,
+    /// Only models that have been used in requests.
+    pub used_only: Option<bool>,
     /// Sort field (name, id, provider, context_limit, input_cost, output_cost).
     pub sort_by: Option<ModelSortBy>,
     /// Sort direction (asc, desc).
@@ -46,6 +48,22 @@ pub(super) async fn handle_get_models(
     let registry = &state.app_state.model_registry;
     let pool = &state.app_state.db_pool;
 
+    // If used_only, fetch the set of models that appear in llm_requests
+    let used_models: Option<std::collections::HashSet<String>> =
+        if params.used_only.unwrap_or(false) {
+            match sqlx::query_scalar::<_, String>(
+                "SELECT DISTINCT model FROM llm_requests",
+            )
+            .fetch_all(pool)
+            .await
+            {
+                Ok(models) => Some(models.into_iter().collect()),
+                Err(_) => Some(std::collections::HashSet::new()),
+            }
+        } else {
+            None
+        };
+
     // Build filter from query params
     let filter = ModelFilter {
         provider: params.provider,
@@ -79,6 +97,14 @@ pub(super) async fn handle_get_models(
 
     for entry in &entries {
         let m_key = entry.model_key;
+
+        // Skip models not in the used set (when used_only is active)
+        if let Some(ref used) = used_models {
+            if !used.contains(m_key) {
+                continue;
+            }
+        }
+
         let m_meta = entry.metadata;
 
         let mut enabled = true;