Phase 23: season history + division zone data + new route

- migrations/0011_season_history.sql: persist one row per completed season
- models/season.rs: SeasonHistoryEntry struct; public SEASON_LENGTH /
  PROMOTION_PTS / RELEGATION_PTS consts; pts_above_safe, can_be_relegated,
  promotion_achievable helpers
- services/season.rs: write history entry on season rollover; get_history()
  returns last 20 seasons newest-first
- routes/division.rs: GET /division now includes promotion_pts, relegation_pts,
  season_length, pts_above_safe, promotion_achievable, can_be_relegated;
  new GET /division/history endpoint
- 3 new integration tests: history empty, history records after promotion,
  division response has zone fields

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app.rs

@@ -201,6 +201,7 @@ pub async fn build(pool: Pool, cfg: Config) -> Result<Router> {
         .route("/settings", get(routes::settings::get_settings))
         .route("/settings", put(routes::settings::put_settings))
         .route("/division", get(routes::division::get_division))
+        .route("/division/history", get(routes::division::get_division_history))
         .route("/achievements", get(routes::achievements::get_achievements))
         .route("/notifications", get(routes::notifications::get_notifications))
         .route("/notifications/read-all", post(routes::notifications::mark_all_notifications_read))