Create a Timestamp for New Account Creation (#24)

# *Timestamp for New Account Creation*

## ♻️ Current situation & Problem
We are transitioning the backend from a fixed, 15min featurization
schedule to a Firestore listener-based schedule (see this
[PR](StanfordHCI/gptcoach-legacy#22) for progress). As
part of this migration, we have set up a listener on the
`studies/STUDY_ID/users` collection to monitor new users. This listener
is not triggered by changes to the users collection, as this would
trigger on every single data upload to any user. Instead, I propose
adding a `created_at` timestamp to each user document, and having the
listener trigger on the query: `new_user_query =
users_ref.where(filter=FieldFilter('created_at', '>',
self.last_query_time))`.

However, this field is not automatically created for new users. This PR
adds functionality to the iOS app to set the field appropriately upon
account creation.

## ⚙️ Release Notes 
* Added a new function `PrismaStandard.setAccountTimestamp()` that
writes the current time to the `created_at` field in the user's document
* Call `standard.setAccountTimestamp()` in the `AccountOnboarding`
continuation

## 📚 Documentation
*Please ensure that you properly document any additions in conformance
to [Spezi Documentation
Guide](https://github.com/StanfordSpezi/.github/blob/main/DOCUMENTATIONGUIDE.md).*
*You can use this section to describe your solution, but we encourage
contributors to document your reasoning and changes using in-line
documentation.*

## ✅ Testing
*Please ensure that the PR meets the testing requirements set by CodeCov
and that new functionality is appropriately tested.*
*This section describes important information about the tests and why
some elements might not be testable.*


## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/CS342/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/CS342/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/CS342/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/CS342/.github/blob/main/CONTRIBUTING.md).

Prisma/Onboarding/AccountOnboarding.swift

@@ -13,6 +13,7 @@ import SwiftUI
 
 struct AccountOnboarding: View {
     @Environment(Account.self) private var account
+    @Environment(PrismaStandard.self) private var standard
     @Environment(OnboardingNavigationPath.self) private var onboardingNavigationPath
 
 
@@ -21,6 +22,7 @@ struct AccountOnboarding: View {
             Task {
                 // Placing the nextStep() call inside this task will ensure that the sheet dismiss animation is
                 // played till the end before we navigate to the next step.
+                await standard.setAccountTimestamp()
                 onboardingNavigationPath.nextStep()
             }
         } header: {

Prisma/Standard/PrismaStandard.swift

@@ -174,6 +174,19 @@ actor PrismaStandard: Standard, EnvironmentAccessible, HealthKitConstraint, Onbo
         }
         try await accountStorage.create(identifier, details)
     }
+
+    func setAccountTimestamp() async {
+        // Add a "created_at" timestamp to the newly created user document
+        let timestamp = Timestamp(date: Date())
+        do {
+            try await self.userDocumentReference.setData([
+                "created_at": timestamp
+            ], merge: true)
+            print("Added timestamp to user document")
+        } catch {
+            print("Error updating document: \(error)")
+        }
+    }
 
     func load(_ identifier: AdditionalRecordId, _ keys: [any AccountKey.Type]) async throws -> PartialAccountDetails {
         guard let accountStorage else {