Fix API contract mismatches with Go backend

- Document.purchasePrice: String? → Double? (matches Go decimal.Decimal)
- TaskTemplate: add regionId/regionName (Go returns these, KMM was ignoring)
- TaskResponse.completions: add comment explaining separate fetch pattern
- Document: add comments clarifying fileUrl vs mediaUrl usage

composeApp/src/commonMain/kotlin/com/tt/honeyDue/models/TaskTemplate.kt

@@ -20,7 +20,9 @@ data class TaskTemplate(
     @SerialName("icon_android") val iconAndroid: String = "",
     val tags: List<String> = emptyList(),
     @SerialName("display_order") val displayOrder: Int = 0,
-    @SerialName("is_active") val isActive: Boolean = true
+    @SerialName("is_active") val isActive: Boolean = true,
+    @SerialName("region_id") val regionId: Int? = null,
+    @SerialName("region_name") val regionName: String? = null
 ) {
     /**
      * Human-readable frequency display