Add chess-style Elo ratings for players

Each player's rating starts at 1500 and updates transactionally with
every finished match: a team's rating is the mean of its two members
and the standard K=32 formula decides the zero-sum delta applied to
both members of a team. Ratings are per game type in a new
player_rating table; match_player records each match's elo_delta.

- GET /api/leaderboard exposes elo and sorts by it
- GET /api/me/matches includes per-player elo deltas
- new GET /api/me/ratings returns the caller's rating per game type
- frontend: Elo column on the leaderboard, per-match delta in the
  history page, current rating in the lobby
- python -m tavolo.backfill_elo recomputes all ratings from the
  recorded match history (one-off backfill for existing matches)

web/src/model.rs

@@ -184,6 +184,10 @@ pub struct MatchPlayer {
     pub seat: usize,
     pub team: String,
     pub won: bool,
+    /// Elo change this match produced for the player; absent for matches
+    /// recorded before ratings existed.
+    #[serde(default)]
+    pub elo_delta: Option<i32>,
 }
 
 #[derive(Debug, Clone, Deserialize)]
@@ -201,6 +205,9 @@ pub struct MatchSummary {
     pub finished_at: String,
     #[serde(default)]
     pub you_won: bool,
+    /// The viewer's Elo change in this match; absent when unrated.
+    #[serde(default)]
+    pub your_elo_delta: Option<i32>,
     #[serde(default)]
     pub players: Vec<MatchPlayer>,
 }
@@ -213,16 +220,38 @@ pub struct MatchesPage {
     pub next_cursor: Option<String>,
 }
 
+fn default_elo() -> i32 {
+    1500
+}
+
 #[derive(Debug, Clone, Deserialize)]
 #[allow(dead_code)]
 pub struct LeaderboardEntry {
     pub user_sub: String,
     pub display_name: String,
+    /// Chess-style Elo rating for the requested game type.
+    #[serde(default = "default_elo")]
+    pub elo: i32,
     pub matches: i32,
     pub wins: i32,
     pub points: i32,
 }
 
+/// The caller's Elo rating for one game type (GET /api/me/ratings).
+#[derive(Debug, Clone, Deserialize)]
+#[allow(dead_code)]
+pub struct PlayerRating {
+    pub game_type: String,
+    pub rating: i32,
+    pub matches_played: i32,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct RatingsPage {
+    #[serde(default)]
+    pub results: Vec<PlayerRating>,
+}
+
 #[derive(Debug, Clone, Deserialize)]
 pub struct LeaderboardPage {
     #[serde(default)]