feat(metrics): per-staff performance view in dashboard

GET /v1/metrics/staff returns bookings, completed, cancelled,
utilization, and revenue per staff member over a trailing window
(default 30d, max 365d). Appointment bookings only — class sessions
have no staff assignment. All active staff appear, including
zero-booking rows.

Staff page gains a localized Performance panel; demo mode seeds
trailing-window bookings and matching metric rows.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/openapi/bookra.openapi.yaml

@@ -1431,6 +1431,34 @@ paths:
                 $ref: "#/components/schemas/ActivityFeedResponse"
         "404":
           description: Tenant membership not found
+  /v1/metrics/staff:
+    get:
+      tags: [Metrics]
+      operationId: getStaffMetrics
+      security:
+        - bearerAuth: []
+      summary: Per-staff appointment metrics over a trailing window
+      description: >-
+        Bookings, completions, cancellations, utilization, and revenue per
+        staff member. Only appointment bookings carry a staff assignment —
+        class attendance is excluded by design. Every active staff member
+        appears, including zero rows.
+      parameters:
+        - name: days
+          in: query
+          schema:
+            type: integer
+            default: 30
+            maximum: 365
+      responses:
+        "200":
+          description: Per-staff metrics
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/StaffMetricsResponse"
+        "404":
+          description: Tenant membership not found
   /v1/export/bookings.csv:
     get:
       tags: [Export]
@@ -2158,6 +2186,42 @@ components:
           type: array
           items:
             $ref: "#/components/schemas/ActivityEvent"
+    StaffMetric:
+      type: object
+      required: [staffId, displayName, bookings, completed, cancelled, utilizationPercent, revenueCents]
+      properties:
+        staffId:
+          type: string
+          format: uuid
+        displayName:
+          type: string
+        bookings:
+          type: integer
+        completed:
+          type: integer
+        cancelled:
+          type: integer
+        utilizationPercent:
+          type: integer
+          description: Share of bookings that stayed confirmed, 0-100
+        revenueCents:
+          type: integer
+          format: int64
+          description: Sum of linked service prices for non-cancelled bookings
+    StaffMetricsResponse:
+      type: object
+      required: [from, to, staff]
+      properties:
+        from:
+          type: string
+          format: date-time
+        to:
+          type: string
+          format: date-time
+        staff:
+          type: array
+          items:
+            $ref: "#/components/schemas/StaffMetric"
     DashboardBookingRequest:
       type: object
       required: [customerName, customerEmail, startsAt]