Fix OpenAPI types of UNIX timestamp fields

[nscuro]

Description

Fields of type Date that are not explicitly serialized with Iso8601DateSerializer are automatically serialized as UNIX epoch timestamp in milliseconds. However the OpenAPI spec declared them to be of type string in date-time format, which is incorrect.

Instead of changing the field's actual type, causing a breaking change, provide a type hint to the OpenAPI generator.

Addressed Issue

Fixes #1276 Fixes #2591

Additional Details

Checklist

• [x] I have read and understand the contributing guidelines
• ~This PR fixes a defect, and I have provided tests to verify that the fix is effective~
• [x] This PR implements an enhancement, and I have provided tests to verify that it works as intended
• ~This PR introduces changes to the database model, and I have added corresponding update logic~
• ~This PR introduces new or alters existing behavior, and I have updated the documentation accordingly~

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -1.00%)	:white_check_mark: ∅ (target: 70.00%)

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (7573f394d7892c913e2b46d38f2cbbd901bc806c) | 21648 | 16402 | 75.77% | | | Head commit (a7119e8d237334099edeeabad689880157466f2b) | 21648 (+0) | 16402 (+0) | 75.77% (**+0.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#3731) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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