Migrate GroupMembershipBundle to SearchResult per ADR-019

[jeremi]

Context

PR #280 review identified that the membership history endpoint returns a SearchResult wrapper but internally uses a different pattern than other search endpoints. Per ADR-019, all list/search responses should use SearchResult consistently.

Current Behavior

The /Group/{id}/membership-history endpoint:

• Builds history entries from spp.group.membership records in Python
• Returns a SearchResult wrapper via create_search_result()
• Each entry is a plain dict, not a typed schema

Desired Behavior

• Define a MembershipHistoryEntry schema (or similar)
• Ensure the response structure is consistent with ADR-019 SearchResult conventions
• Consider whether GroupMembershipBundle schema should be replaced with standard SearchResult

Scope

This is a larger refactoring that affects:

• spp_api_v2/schemas/ - schema definitions
• spp_api_v2/routers/group.py - endpoint response model
• spp_api_v2/services/group_service.py - return types
• Tests

Source

Gemini code review finding on PR #280, triaged as valid but deferred to separate PR.