Add MilestoneAnswers

[lkeegan]

• add models
  • MilestoneAnswerSession
  • this collects the milestone answers for a given child, user and timeframe
  • MilestoneAnswer
• add endpoints
  • get /milestone-answers/{child_id}
  • returns current MilestoneAnswerSession (or a new one if most recent one is > 7 days old)
  • put /milestone-answers/{milestone_answer_session_id}
  • submit an answer to a milestone question for the specified MilestoneAnswerSession
  • overwrites any previous answer for this question in this MilestoneAnswerSession
• update Milestone component accordingly
  • take a MilestoneAnswerSession as a prop
  • submits answers to the endpoint as the user provides them
• update milestone preview route
  • uses as data the first milestone group and the current answer session for the first child of the user
• add missing tests for child endpoints and fix revealed bugs
• resolves #110

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.00000% with 117 lines in your changes missing coverage. Please review.

Project coverage is 17.22%. Comparing base (5e2c9bc) to head (5cc3927).

Files with missing lines	Patch %	Lines
frontend/src/lib/client/schemas.gen.ts	0.00%	44 Missing :warning:
frontend/src/lib/client/services.gen.ts	0.00%	25 Missing :warning:
frontend/src/lib/components/Milestone.svelte	0.00%	24 Missing :warning:
frontend/src/routes/milestone/+page.svelte	0.00%	24 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #116 +/- ## ========================================== + Coverage 16.23% 17.22% +0.99% ========================================== Files 98 98 Lines 4829 4974 +145 Branches 125 125 ========================================== + Hits 784 857 +73 - Misses 3975 4047 +72 Partials 70 70 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
7 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud