Return age-appriopriate Milestones

[lkeegan]

• /milestone-groups endpoint now requires a child_id
• it returns only the milestones with expected age within +/-6 months of the child age for each group
• remove previous endpoint that returned a single milestone group by id as it is not needed
• refactor children test fixture to reduce duplication
• add some missing type annotations
• update demo milestone route in frontend accordingly
• resolves #132

depends on #140

[sonarcloud[bot]]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 26.34%. Comparing base (9310ea8) to head (ccb8501).

Files with missing lines	Patch %	Lines
frontend/src/routes/milestone/+page.svelte	0.00%	4 Missing :warning:
...src/lib/components/Admin/EditMilestoneModal.svelte	0.00%	2 Missing :warning:
frontend/src/lib/components/UserLandingPage.svelte	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #142 +/- ## ========================================== + Coverage 26.05% 26.34% +0.28% ========================================== Files 92 92 Lines 3500 3519 +19 Branches 121 121 ========================================== + Hits 912 927 +15 - Misses 2522 2526 +4 Partials 66 66 ```

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