Show an activity entry with no operator as System (#138)

[theobong]

Part of civfix/issue-tracker#138

What changed

In the admin activity feed, an entry with no operator behind it now shows "System" instead of "Operator", and an anonymous suggestion of a city contact shows "A neighbor". "Operator" stays only for an operator with no display name. No admin screen shows this feed yet. Admin.

Before you start

• Where: staging (admin.civfix.dev) after the main push
• Sign in as: operator

Verify

No admin screen reads the activity feed; "Not covered" gives the API check.

Regression

No admin screen or app flow reads the activity feed, so there is no flow to sweep.

Not covered

• The feed on staging (API check, not a UI step): signed in to admin.civfix.dev as an operator, open /v1/admin/activity — Expect: entries the system wrote, such as "Sent an outreach digest" or "A send to a jurisdiction failed", show "System" as who; entries by a named operator still show that operator's name.
• An anonymous city-contact suggestion: no app sends one yet. Checked by the unit and database suites.

[greptile-apps]

Confidence Score: 5/5

Safe to merge.

What we checked:

• Inserted audit rows with both absent and present actors, and loaded them through the activity repository and classifier.
• Updated the code to project absent actors as actorless: true, display absent actorless activity as System, show an actorless public contact suggestion as A neighbor, and retain actorless: false for the named actor.
• Ran the focused admin activity unit test suite, which completed with 12 passing tests.
• Uploaded the exact authored probe script and both captured command outputs to document the actorless projections and validation results.

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