Implement Member Activities Aggregation

[StatPan]

Phase: 3 - Complex/Uncertain
Priority: Medium

Why Last: Requires aggregating 3+ APIs, uncertain if identifiers align.

Tasks:

• Research if APIs share common member identifier
• Map Service IDs (표결정보, 발의법률안, 출석정보)
• get_member_activities(member_name) - aggregate votes, bills, attendance
• Handle identifier mismatches gracefully
• Tests

Acceptance Criteria:

• Returns aggregated activities OR clear error if not feasible
• Fallback to individual API calls if aggregation fails

Risk: High - might not be implementable if APIs use different naming.

Related to: #11