feat(match): atomic exactly-once match-completion transaction

Add complete_match: one BEGIN/COMMIT that validates identity + result,
enforces a durable (profile_id, match_identity) uniqueness guard
(migration 0022 match_completions), persists match history, and grants
coins + XP/level-ups + W/D/L/DNF statistics + objectives + achievements
exactly once. Any failure rolls the whole match back (no compensating
cleanup). Handles sequential/restart/concurrent replay, conflicting
re-report (first result canonical), DNF (loss economics, own stat
bucket) and no-contest (zero economic effect).

Adds tx-scoped variants: statistics::record_match_tx/
record_position_goals_tx, objective::increment_metric_tx,
achievement::check_and_unlock_tx. New MatchResultKind/CompleteMatchRequest/
MatchCompletionResult models + POST /matches/complete route.

src/services/statistics.rs

@@ -1,6 +1,7 @@
 use crate::{db::Pool, error::AppResult, models::statistics::Statistics};
+use sqlx::{Sqlite, Transaction};
 
-const SELECT_STATS: &str = "SELECT profile_id, matches_played, matches_won, matches_drawn, matches_lost, goals_scored, goals_conceded, packs_opened, sbcs_completed, total_coins_earned, win_streak, best_win_streak, updated_at FROM statistics WHERE profile_id = ?";
+const SELECT_STATS: &str = "SELECT profile_id, matches_played, matches_won, matches_drawn, matches_lost, matches_dnf, goals_scored, goals_conceded, packs_opened, sbcs_completed, total_coins_earned, win_streak, best_win_streak, updated_at FROM statistics WHERE profile_id = ?";
 
 pub async fn get_or_create(pool: &Pool, profile_id: &str) -> AppResult<Statistics> {
     if let Some(s) = sqlx::query_as::<_, Statistics>(SELECT_STATS)
@@ -77,6 +78,77 @@ pub async fn record_match(
     Ok(())
 }
 
+/// Record a completed match within an existing transaction (the atomic
+/// match-completion path). `outcome` is `win` | `draw` | `loss` | `dnf`. A DNF
+/// (abandon/quit) increments its own bucket — never `matches_lost` — and, like a
+/// loss, resets the win streak. All-or-nothing with the caller's transaction; it
+/// never commits on its own, so a later failure rolls this back with everything
+/// else.
+pub async fn record_match_tx(
+    tx: &mut Transaction<'_, Sqlite>,
+    profile_id: &str,
+    outcome: &str,
+    goals_for: i64,
+    goals_against: i64,
+    coins: i64,
+    now: &str,
+) -> AppResult<()> {
+    sqlx::query("INSERT OR IGNORE INTO statistics (profile_id, updated_at) VALUES (?, ?)")
+        .bind(profile_id)
+        .bind(now)
+        .execute(&mut **tx)
+        .await?;
+
+    let current_streak: i64 =
+        sqlx::query_scalar("SELECT win_streak FROM statistics WHERE profile_id = ?")
+            .bind(profile_id)
+            .fetch_one(&mut **tx)
+            .await?;
+
+    let (w, d, l, dnf) = match outcome {
+        "win" => (1i64, 0i64, 0i64, 0i64),
+        "draw" => (0, 1, 0, 0),
+        "dnf" => (0, 0, 0, 1),
+        _ => (0, 0, 1, 0),
+    };
+    let new_streak = if outcome == "win" {
+        current_streak + 1
+    } else {
+        0
+    };
+
+    sqlx::query(
+        "UPDATE statistics SET
+            matches_played     = matches_played + 1,
+            matches_won        = matches_won + ?,
+            matches_drawn      = matches_drawn + ?,
+            matches_lost       = matches_lost + ?,
+            matches_dnf        = matches_dnf + ?,
+            goals_scored       = goals_scored + ?,
+            goals_conceded     = goals_conceded + ?,
+            total_coins_earned = total_coins_earned + ?,
+            win_streak         = ?,
+            best_win_streak    = MAX(best_win_streak, ?),
+            updated_at         = ?
+         WHERE profile_id = ?",
+    )
+    .bind(w)
+    .bind(d)
+    .bind(l)
+    .bind(dnf)
+    .bind(goals_for)
+    .bind(goals_against)
+    .bind(coins)
+    .bind(new_streak)
+    .bind(new_streak)
+    .bind(now)
+    .bind(profile_id)
+    .execute(&mut **tx)
+    .await?;
+
+    Ok(())
+}
+
 pub async fn increment_packs_opened(pool: &Pool, profile_id: &str) -> AppResult<()> {
     get_or_create(pool, profile_id).await?;
     let now = chrono::Utc::now().to_rfc3339();
@@ -121,6 +193,26 @@ pub async fn record_position_goals(
     Ok(())
 }
 
+/// Transaction-scoped [`record_position_goals`] for the atomic match-completion
+/// path.
+pub async fn record_position_goals_tx(
+    tx: &mut Transaction<'_, Sqlite>,
+    profile_id: &str,
+    positions: &[String],
+) -> AppResult<()> {
+    for position in positions {
+        sqlx::query(
+            "INSERT INTO position_goals (profile_id, position, goals) VALUES (?, ?, 1) \
+             ON CONFLICT(profile_id, position) DO UPDATE SET goals = goals + 1",
+        )
+        .bind(profile_id)
+        .bind(position)
+        .execute(&mut **tx)
+        .await?;
+    }
+    Ok(())
+}
+
 pub async fn get_position_goals(pool: &Pool, profile_id: &str) -> AppResult<Vec<(String, i64)>> {
     let rows: Vec<(String, i64)> = sqlx::query_as(
         "SELECT position, goals FROM position_goals WHERE profile_id = ? ORDER BY goals DESC",