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/api.rs

@@ -91,6 +91,18 @@ pub async fn my_matches(cursor: Option<&str>) -> Result<MatchesPage, String> {
     resp.json().await.map_err(|e| e.to_string())
 }
 
+/// Fetch the caller's Elo ratings (one row per game type played).
+pub async fn my_ratings() -> Result<RatingsPage, String> {
+    let resp = Request::get("/api/me/ratings")
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    if !resp.ok() {
+        return Err(server_error(resp.status()));
+    }
+    resp.json().await.map_err(|e| e.to_string())
+}
+
 pub async fn leaderboard() -> Result<LeaderboardPage, String> {
     let resp = Request::get("/api/leaderboard")
         .send()

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)]

web/src/pages/history.rs

@@ -72,6 +72,10 @@ pub fn HistoryPage() -> View {
                                 .collect::<Vec<_>>()
                                 .join(" & ");
                             let outcome = if m.you_won { "Won" } else { "Lost" };
+                            let elo_delta = m
+                                .your_elo_delta
+                                .map(|d| if d >= 0 { format!("+{d}") } else { d.to_string() })
+                                .unwrap_or_else(|| "—".to_string());
                             view! {
                                 tr {
                                     td { (m.finished_at.replace('T', " ").chars().take(16).collect::<String>()) }
@@ -80,6 +84,9 @@ pub fn HistoryPage() -> View {
                                     td { (m.team_a_score) " – " (m.team_b_score) }
                                     td { "Team " (m.winner_team) }
                                     td(class=if m.you_won { "won" } else { "lost" }) { (outcome) }
+                                    td(class=if m.you_won { "won" } else { "lost" }) {
+                                        (elo_delta)
+                                    }
                                 }
                             }
                         })
@@ -94,6 +101,7 @@ pub fn HistoryPage() -> View {
                                     th { "Score" }
                                     th { "Winner" }
                                     th { "You" }
+                                    th { "Elo" }
                                 }
                             }
                             tbody { (table_rows) }

web/src/pages/leaderboard.rs

@@ -39,6 +39,7 @@ pub fn LeaderboardPage() -> View {
                                 tr {
                                     td { (i + 1) }
                                     td { (e.display_name.clone()) }
+                                    td { (e.elo) }
                                     td { (e.wins) }
                                     td { (e.matches) }
                                     td { (e.points) }
@@ -52,6 +53,7 @@ pub fn LeaderboardPage() -> View {
                                 tr {
                                     th { "#" }
                                     th { "Player" }
+                                    th { "Elo" }
                                     th { "Wins" }
                                     th { "Matches" }
                                     th { "Points" }

web/src/pages/lobby.rs

@@ -20,6 +20,8 @@ fn fallback_game_types() -> Vec<GameTypeInfo> {
 pub fn LobbyPage() -> View {
     // Outer None = still loading; Some(None) = logged out.
     let user = create_signal(Option::<Option<User>>::None);
+    // The player's Elo rating for the first rated game; None while loading.
+    let rating = create_signal(Option::<i32>::None);
     let error = create_signal(Option::<String>::None);
     let code = create_signal(String::new());
     let game_types = create_signal(fallback_game_types());
@@ -28,7 +30,17 @@ pub fn LobbyPage() -> View {
 
     spawn_local(async move {
         match api::me().await {
-            Ok(me) => user.set(Some(me)),
+            Ok(me) => {
+                if me.is_some() {
+                    spawn_local(async move {
+                        if let Ok(p) = api::my_ratings().await {
+                            // Unrated players sit at the initial 1500.
+                            rating.set(Some(p.results.first().map(|r| r.rating).unwrap_or(1500)));
+                        }
+                    });
+                }
+                user.set(Some(me));
+            }
             Err(e) => {
                 error.set(Some(e));
                 user.set(Some(None));
@@ -88,7 +100,12 @@ pub fn LobbyPage() -> View {
                 Some(Some(me)) => view! {
                     div(class="lobby-grid") {
                         nav(class="top-nav") {
-                            span(class="whoami") { "Signed in as " strong { (me.name.clone()) } }
+                            span(class="whoami") {
+                                "Signed in as " strong { (me.name.clone()) }
+                                (rating.get_clone().map(|r| view! {
+                                    span(class="rating") { " · Elo " (r) }
+                                }))
+                            }
                             a(href="/history") { "My matches" }
                             a(href="/leaderboard") { "Leaderboard" }
                             a(href="/auth/logout", rel="external") { "Log out" }

web/style.css

@@ -49,6 +49,10 @@ body {
     margin-right: auto;
 }
 
+.whoami .rating {
+    color: var(--muted, #888);
+}
+
 .panel {
     background: var(--panel);
     border-radius: 12px;