feat(seer): Add single-project Seer settings endpoint

[srest2021]

Relates to CW-1285

Adds an endpoint for managing per-project Seer settings:

• GET /api/0/projects/{org}/{project}/seer/settings/ — single-project settings read
• PUT /api/0/projects/{org}/{project}/seer/settings/ — single-project settings update

Uses the update_seer_project_settings helper from #115037 to translate high-level fields (agent, integrationId, stoppingPoint, scannerAutomation) into project options.

Bulk endpoint followup: #115234

Example: GET

GET /api/0/projects/sentry/test-seer-settings/seer/settings/

{
    "projectId": "2",
    "projectSlug": "test-seer-settings",
    "agent": "seer",
    "integrationId": null,
    "stoppingPoint": "off",
    "scannerAutomation": true,
    "reposCount": 0
}

{
    "projectId": "2",
    "projectSlug": "test-seer-settings",
    "agent": "cursor_background_agent",
    "integrationId": "42",
    "stoppingPoint": "open_pr",
    "scannerAutomation": false,
    "reposCount": 3
}

Example: PUT

Update stopping point:

PUT /api/0/projects/sentry/test-seer-settings/seer/settings/

{"stoppingPoint": "open_pr"}

{
    "projectId": "2",
    "projectSlug": "test-seer-settings",
    "agent": "seer",
    "integrationId": null,
    "stoppingPoint": "open_pr",
    "scannerAutomation": true,
    "reposCount": 1
}

Update agent to external coding agent:

PUT /api/0/projects/sentry/test-seer-settings/seer/settings/

{"agent": "cursor_background_agent", "integrationId": 42}

{
    "projectId": "2",
    "projectSlug": "test-seer-settings",
    "agent": "cursor_background_agent",
    "integrationId": "42",
    "stoppingPoint": "open_pr",
    "scannerAutomation": true,
    "reposCount": 1
}

Turn off automation:

PUT /api/0/projects/sentry/test-seer-settings/seer/settings/

{"stoppingPoint": "off"}

{
    "projectId": "2",
    "projectSlug": "test-seer-settings",
    "agent": "seer",
    "integrationId": null,
    "stoppingPoint": "off",
    "scannerAutomation": true,
    "reposCount": 1
}

[linear-code]

CW-1285

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.46%

[JoshFerge]

couple comments, but looks good 🙏🏼 thanks for splitting out