Retrospection data model and store

[sunba91-su]

Goal

Add database tables and store methods for team retrospectives.

Acceptance Criteria

• New tables added to schema migration:
  • retro_sessions(id TEXT PK, team_id TEXT NOT NULL REFERENCES teams(id), date TEXT NOT NULL, status TEXT NOT NULL DEFAULT "open", created_at TIMESTAMP)
  • retro_columns(session_id TEXT NOT NULL REFERENCES retro_sessions(id), label TEXT NOT NULL, sort_order INT NOT NULL, UNIQUE(session_id, label))
  • retro_items(id TEXT PK, session_id TEXT NOT NULL REFERENCES retro_sessions(id), column_id TEXT NOT NULL, user_id TEXT NOT NULL, username TEXT NOT NULL, content TEXT NOT NULL, created_at TIMESTAMP, UNIQUE(session_id, user_id, column_id))
• Store methods:
  • CreateRetroSession, GetRetroSession, GetActiveRetroSession, CloseRetroSession
  • AddRetroItem, GetRetroItems (grouped by column), ListRetroSessions
• Define Go structs: RetroSession, RetroColumn, RetroItem
• Test methods in store_test.go

Technical Notes

• Follow patterns from standup_sessions / standup_responses in store
• Retro columns are flexible (defaults: Went Well, To Improve, Action Items)
• Items are per-user-per-column

Dependencies

• None (pure data layer)