Add daily usage metrics (CPU-minutes, RAM GB-minutes)

Introduce pre-computed daily usage rollups from sandbox_metrics_snapshots.
An hourly background worker aggregates completed days, while today's
usage is computed live from snapshots at query time for freshness.

Backend: new daily_usage table, rollup worker, UsageService, and
GET /v1/capsules/usage endpoint with date range filtering (up to 92 days).

Frontend: replace Usage page placeholder with bar charts (Chart.js),
summary total cards, and preset/custom date range controls.

internal/api/openapi.yaml

@@ -921,6 +921,38 @@ paths:
         "400":
           $ref: "#/components/responses/BadRequest"
 
+  /v1/capsules/usage:
+    get:
+      summary: Get daily CPU and RAM usage for your team
+      operationId: getCapsuleUsage
+      tags: [capsules]
+      security:
+        - apiKeyAuth: []
+      parameters:
+        - name: from
+          in: query
+          required: false
+          schema:
+            type: string
+            format: date
+          description: Start date (YYYY-MM-DD). Defaults to 30 days ago.
+        - name: to
+          in: query
+          required: false
+          schema:
+            type: string
+            format: date
+          description: End date (YYYY-MM-DD). Defaults to today.
+      responses:
+        "200":
+          description: Daily usage data for the team
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/UsageResponse"
+        "400":
+          $ref: "#/components/responses/BadRequest"
+
   /v1/capsules/{id}:
     parameters:
       - name: id
@@ -2432,6 +2464,28 @@ components:
             after this duration of inactivity (no exec or ping). 0 means
             no auto-pause.
 
+    UsageResponse:
+      type: object
+      properties:
+        from:
+          type: string
+          format: date
+        to:
+          type: string
+          format: date
+        points:
+          type: array
+          items:
+            type: object
+            properties:
+              date:
+                type: string
+                format: date
+              cpu_minutes:
+                type: number
+              ram_mb_minutes:
+                type: number
+
     CapsuleStats:
       type: object
       properties: