During worker-instance-start, after the provider machine is created,
fetch the authoritative price via EstimateCost() and store it in
cost_per_hour along with the started_at timestamp, replacing the
manual operator-entered price.

During worker-software-install, re-fetch the current provider price
and compare against stored cost_per_hour. Log a warning if it changed.

During worker-instance-shutdown, verify the price one final time,
calculate total wall-clock runtime as shutdown_at - started_at, and
store the computed total_cost in the new column.

Adds DB migration 00195 for started_at and total_cost columns on
vision_worker, along with matching jet model/table field additions.
Updates computeUptime to prefer started_at over created_at.

@@ -0,0 +1,7 @@
+-- +goose Up
+ALTER TABLE vision_worker ADD COLUMN started_at TIMESTAMP;

@@ -211,0 +211,4 @@
+// it is used as the reference start time; otherwise createdAt is used.
+func computeUptime(createdAt time.Time, startedAt, shutdownAt *time.Time) float64 {
+	s := startedAt
+	if s == nil {

@@ -78,0 +83,4 @@
+	}
+	if worker == nil {
+		log.Printf("worker-instance-shutdown: worker %d not found during shutdown", workerID)
+		return nil

@@ -78,0 +89,4 @@
+	// Determine the reference start time for cost calculation
+	startTime := worker.StartedAt
+	if startTime == nil {
+		startTime = &worker.CreatedAt

@@ -78,0 +93,4 @@
+	}
+	if startTime == nil {
+		// Fallback: use creation time
+		created := worker.CreatedAt