Add /-/tasks introspection endpoint for supervised background tasks

[asg017]

Adds new /-/tasks and /-/tasks.json endpoints for monitoring new "background tasks" introduced in #2889 , similar to /-/threads. It requires the permissions-debug permission like /-/threads and friends.

$ curl http://localhost:8010/-/tasks.json
{
  "ok": true,
  "tasks": [
    {
      "name": "datasette-cron",
      "state": "running",
      "plugin": "cron",
      "started_at": "2026-09-02T16:40:07.082274+00:00",
      "exception": null
    }
  ],
  "launched": true
}

🤖 Claude-generated PR description

Sixth and final PR in the startup/lifecycle stack, on top of #2891.

What this does

Adds a /-/tasks (and /-/tasks.json) introspection endpoint listing every supervised background task registered via datasette.add_background_task() (#2889), plus whether the launch phase has run.

Changes

• datasette/app.py: new _tasks() data method exposed through JsonDataView at /-/tasks, guarded by the existing permissions-debug permission (same as /-/threads). Each task reports name, state, plugin, started_at, and a repr() of its exception if it crashed.
• datasette/background_tasks.py: public launched property on the supervisor so the endpoint can distinguish "no tasks registered" from "registered but launch hasn't fired yet".
• docs/introspection.rst and docs/json_api.rst: documents the endpoint.

Docs

Rolled down from the former docs-only tip PR (#2893), this PR now also carries the stack's capstone docs:

• docs/introspection.rst: /-/tasks docs cross-reference the background-task API and lifecycle sections.
• docs/internals.rst: the API reference sections mention /-/tasks where relevant.
• docs/changelog.rst: the full 1.0a39 release notes for the whole stack — single-event-loop serve, lifespan startup, the background-tasks API, the shutdown() hook, the asgi_wrapper reorder, /-/tasks — including the "Migrating away from asgi_wrapper bootstrap hacks" guide.

Tests

New tests/test_tasks_endpoint.py covering the payload shape, permission gating, and states; /-/tasks added to the endpoint inventories in tests/test_permissions.py and tests/test_success_envelope.py.

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (c2f7735) to head (66b5291).

Files with missing lines	Patch %	Lines
datasette/app.py	0.00%	3 Missing ⚠️
datasette/background_tasks.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           asg017/first-request-5-wrapper-reorder   #2892   +/-   ##
======================================================================
  Coverage                                    0.00%   0.00%           
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  Files                                          74      74           
  Lines                                       12461   12467    +6     
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- Misses                                      12461   12467    +6     

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