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>

migrations/0011_season_history.sql

@@ -0,0 +1,18 @@
+-- Season history: one row per completed season
+CREATE TABLE IF NOT EXISTS season_history (
+    id          TEXT PRIMARY KEY,
+    profile_id  TEXT NOT NULL,
+    season_number INTEGER NOT NULL,
+    division    INTEGER NOT NULL,
+    season_points INTEGER NOT NULL,
+    wins        INTEGER NOT NULL,
+    draws       INTEGER NOT NULL,
+    losses      INTEGER NOT NULL,
+    result      TEXT NOT NULL,   -- 'promoted' | 'maintained' | 'relegated'
+    new_division INTEGER NOT NULL,
+    coins_awarded INTEGER NOT NULL,
+    pack_awarded TEXT,            -- nullable
+    ended_at    TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_season_history_profile ON season_history (profile_id, season_number DESC);