feat: add created_at feed filters

[mackenziereading19]

Summary:

Adds created_after and created_before query parameters to the feed-list endpoints so API consumers can filter feeds based on when they were added to the Mobility Database.

This addresses #874, where stakeholders want to track newly added feeds. The created_after / created_before naming follows the existing downloaded_after / downloaded_before API pattern.

The filters are available on:

• /v1/feeds
• /v1/gtfs_feeds
• /v1/gtfs_rt_feeds
• /v1/gbfs_feeds

Expected behavior:

created_after and created_before accept ISO date/time values and apply inclusive bounds:

• created_after → created_at >= value
• created_before → created_at <= value

They can be supplied independently or together to define a range. Invalid date values return 422.

No database migration is required because created_at already exists on feed records.

Testing tips:

Test any of the four feed-list endpoints with:

• ?created_after=<ISO datetime>
• ?created_before=<ISO datetime>
• both parameters together
• an invalid date value to verify the 422 response

Integration tests cover all four endpoints, inclusive boundary behavior, combined ranges, and invalid values.

Local verification:

• Full feed API integration suite: 111 passed

• Broader relevant feed tests: 144 passed

• Focused created_at tests: 16 passed

• Black: passed

• git diff --check: passed

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything

• Add or update any needed documentation to the repo

• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification

• Linked all relevant issues

• Include screenshot(s) showing how this pull request works and fixes the issue(s)

Closes #874

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[mackenziereading19]

Added screenshots from the local Swagger UI running this PR against the populated test database.

1. GET /v1/feeds exposes the new created_after and created_before query parameters.
2. A request using created_after=2026-08-30T19:47:40.620207Z returns HTTP 200 and includes gbfs-system_id_3, whose created_at is exactly 2026-08-30T19:47:40.620207Z, demonstrating the inclusive lower bound.
3. An invalid created_after value returns HTTP 422 with the expected ISO 8601 validation message.

The automated integration tests additionally cover both bounds, combined ranges, invalid values, and all four feed-list endpoints.