feat: Implement Player Retention Analytics System (Phase 2.2)

Backend:
- Add player_sessions table (migration 004) for session tracking
- Implement retention calculation queries (24h/48h/72h post-wipe)
- Add /api/plugin/player-event endpoint for join/leave tracking
- Add /api/analytics/retention endpoint with CSV export
- Track unique players, session duration, new vs returning ratio

Frontend:
- Create PlayerRetentionView with ECharts retention curves
- Add multi-wipe comparison (last 3/6/10/20 wipes)
- Display summary metrics and detailed wipe table
- Add /retention route to router

Plugin:
- Update CorrosionCompanion.cs to send player events to new endpoint
- Track player join/leave with license_key authentication

Enables data-driven wipe timing optimization by answering:
"What percentage of players return 24h/48h/72h after a wipe?"

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/api/mod.rs

@@ -15,3 +15,5 @@ pub mod early_access;
 pub mod admin;
 pub mod ws;
 pub mod analytics;
+pub mod plugin;
+pub mod settings;