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>

CHANGELOG.md

@@ -4,6 +4,40 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+### Added (Phase 2.2 — Player Retention Analytics)
+
+**Backend:**
+- Migration `004_player_sessions.sql` — Player session tracking table with indexes for retention queries
+- `backend/src/db/player_sessions.rs` — Complete player session tracking and retention analysis:
+  - `track_player_join()` / `track_player_leave()` — Record individual player sessions
+  - `calculate_retention_after_wipe()` — Calculate 24h/48h/72h return rates per wipe
+  - `get_unique_player_count()` / `get_avg_session_duration()` — Session metrics
+  - `get_new_vs_returning_ratio()` — New vs returning player analysis
+  - `get_recent_wipe_retention_metrics()` — Multi-wipe retention trends
+  - `cleanup_old_player_sessions()` — 90-day retention cleanup
+- `backend/src/api/plugin.rs` — Plugin event endpoints:
+  - `POST /api/plugin/player-event` — Track player join/leave events
+  - `POST /api/plugin/checkin` — Plugin registration on server start
+- Extended `backend/src/api/analytics.rs` with retention endpoints:
+  - `GET /api/analytics/retention?wipe_count=6` — Multi-wipe retention metrics
+  - `GET /api/analytics/retention/export` — CSV export of retention data
+
+**Frontend:**
+- `PlayerRetentionView.vue` — Complete retention analytics dashboard:
+  - ECharts retention curve (24h/48h/72h lines across multiple wipes)
+  - Summary cards: unique players, avg session duration, new vs returning ratio
+  - Wipe selector (last 3/6/10/20 wipes)
+  - Detailed wipe table with retention percentages
+  - CSV export functionality
+- Added route `/retention` to router
+- TypeScript interfaces: `WipeRetentionMetric`, `SessionSummary`, `RetentionResponse`
+
+**Plugin:**
+- Updated `CorrosionCompanion.cs` to track player events via `/api/plugin/player-event`
+- Modified `OnPlayerConnected` / `OnPlayerDisconnected` hooks with license_key authentication
+
+**Purpose:** Answers critical question: "What percentage of players return 24h/48h/72h after a wipe?" Enables data-driven wipe timing optimization and player retention analysis.
+
 ### Added (Phase 2.2 — Map Analytics System)
 
 **Backend:**