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/migrations/004_player_sessions.sql

@@ -0,0 +1,26 @@
+-- Player Session Tracking for Retention Analytics
+-- Phase 2 Feature: Track individual player sessions to calculate post-wipe retention
+
+CREATE TABLE player_sessions (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    license_id UUID NOT NULL REFERENCES licenses(id) ON DELETE CASCADE,
+    steam_id VARCHAR(20) NOT NULL,
+    player_name VARCHAR(100) NOT NULL,
+    session_start TIMESTAMPTZ NOT NULL,
+    session_end TIMESTAMPTZ,
+    duration_seconds INTEGER,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_player_sessions_license ON player_sessions(license_id);
+CREATE INDEX idx_player_sessions_steam ON player_sessions(license_id, steam_id);
+CREATE INDEX idx_player_sessions_start ON player_sessions(session_start DESC);
+CREATE INDEX idx_player_sessions_steam_start ON player_sessions(license_id, steam_id, session_start DESC);
+
+-- Index for retention queries (JOIN with wipe_history on date ranges)
+CREATE INDEX idx_player_sessions_license_start_range ON player_sessions(license_id, session_start)
+    WHERE session_start IS NOT NULL;
+
+COMMENT ON TABLE player_sessions IS 'Individual player join/leave sessions for retention analytics';
+COMMENT ON COLUMN player_sessions.duration_seconds IS 'Calculated when session_end is set (session_end - session_start)';
+COMMENT ON INDEX idx_player_sessions_steam_start IS 'Optimizes "last session for steam_id" queries when player leaves';