DEV: add participant regions to program analytics

[TThanos3000]

Audit result

• User.city remains the only persisted user geography field used by this change.
• The existing manager overview exposes project regions only; participant-region aggregation was absent.
• Participant regions therefore require this separate backend contract before the Angular UI can consume them.

Summary

• adds summary.participant_regions to GET /programs/<program_id>/manager-overview/ using the existing regions response shape;
• uses the same authoritative participant population as summary.participants.total: unique non-null PartnerProgramUserProfile.user_id values for the selected program;
• excludes null, empty and whitespace-only User.city values;
• trims surrounding whitespace, groups equal resulting strings and sorts by count descending, then name;
• preserves legacy values exactly as separate analytics entries: no lowercasing, title-casing, typo correction, city-to-region mapping or data migration.

Contract

{
  "summary": {
    "regions": {
      "total": 1,
      "items": [{"name": "Москва", "count": 2}]
    },
    "participant_regions": {
      "total": 2,
      "items": [
        {"name": "Москва", "count": 2},
        {"name": "Набережные Челны", "count": 1}
      ]
    }
  }
}

summary.regions keeps its existing project-region semantics. summary.participant_regions contains user geography from User.city.

Verification

• targeted manager analytics tests: 17/17 passed;
• full local backend suite: 581/581 passed (1 skipped);
• GitHub Backend PostgreSQL CI: passed, including an empty-database migration run and the full backend suite;
• GitHub standard CI tests: passed;
• GitHub CI lint: passed;
• Black on changed Python files: passed;
• flake8 on changed Python files: passed;
• repository-wide flake8 still reports pre-existing violations in unrelated, unchanged files;
• python manage.py check: passed;
• python manage.py check --tag models: passed;
• python manage.py makemigrations --check --dry-run: no changes detected;
• git diff --check: passed.

Scope

• no models or migrations;
• no changes to User.city data;
• no Angular or React changes;
• no workflow, Docker or deploy changes.

Draft only. Do not merge or deploy automatically.