feat(flags): Set up initial backend for feature management

[havenbarnes]

Problem

#26955

Follow up to https://github.com/PostHog/posthog/pull/26363/files

Changes

Adds the following endpoints for feature management CRUD ops (from show_urls):

GET /api/projects/<parent_lookup_project_id>/features/ 
GET, POST, PUT, PATCH /api/projects/<parent_lookup_project_id>/features/<pk>/
GET /api/projects/<parent_lookup_project_id>/features/<pk>/alerts/ 
GET /api/projects/<parent_lookup_project_id>/features/<pk>/primary_early_access_feature/    
GET /api/projects/<parent_lookup_project_id>/features/<pk>/primary_early_access_feature/

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Added API tests

[posthog-bot]

📸 UI snapshots have been updated

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• webkit: 0 added, 0 modified, 0 deleted

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[posthog-bot]

📸 UI snapshots have been updated

1 snapshot changes in total. 0 added, 1 modified, 0 deleted:

• chromium: 0 added, 1 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

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[posthog-bot]

📸 UI snapshots have been updated

2 snapshot changes in total. 0 added, 2 modified, 0 deleted:

• chromium: 0 added, 2 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

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[posthog-bot]

📸 UI snapshots have been updated

1 snapshot changes in total. 0 added, 1 modified, 0 deleted:

• chromium: 0 added, 1 modified, 0 deleted (diff for shard 2)
• webkit: 0 added, 0 modified, 0 deleted

Triggered by this commit.

👉 Review this PR's diff of snapshots.

[posthog-bot]

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