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/objective.rs

@@ -8,6 +8,7 @@ use crate::{
 use anyhow::Context;
 use std::path::Path;
 use uuid::Uuid;
+use sqlx::{Sqlite, Transaction};
 
 pub fn load_objective_definitions(data_dir: &str) -> anyhow::Result<Vec<ObjectiveDefinition>> {
     let dir = Path::new(data_dir).join("objectives");
@@ -120,6 +121,72 @@ pub async fn increment_metric(
     Ok(completed_ids)
 }
 
+/// Transaction-scoped [`increment_metric`] for the atomic match-completion path.
+/// Same semantics, but every read/write runs inside the caller's transaction so
+/// objective progress commits (or rolls back) together with the coins, XP, and
+/// statistics of the same match. `now` is threaded so one match stamps a single
+/// timestamp.
+pub async fn increment_metric_tx(
+    tx: &mut Transaction<'_, Sqlite>,
+    profile_id: &str,
+    defs: &[ObjectiveDefinition],
+    metric: &str,
+    amount: i64,
+    now: &str,
+) -> AppResult<Vec<String>> {
+    let mut completed_ids = Vec::new();
+
+    for def in defs.iter().filter(|d| d.metric.as_str() == metric) {
+        let existing = sqlx::query_as::<_, ObjectiveProgress>(
+            "SELECT id, profile_id, objective_id, current, completed, claimed, updated_at FROM objective_progress WHERE profile_id = ? AND objective_id = ?"
+        )
+        .bind(profile_id)
+        .bind(&def.id)
+        .fetch_optional(&mut **tx)
+        .await?;
+
+        if let Some(prog) = existing {
+            if prog.completed {
+                continue;
+            }
+            let new_val = (prog.current + amount).min(def.target);
+            let now_complete = new_val >= def.target;
+            sqlx::query(
+                "UPDATE objective_progress SET current = ?, completed = ?, updated_at = ? WHERE id = ?"
+            )
+            .bind(new_val)
+            .bind(now_complete)
+            .bind(now)
+            .bind(&prog.id)
+            .execute(&mut **tx)
+            .await?;
+            if now_complete {
+                completed_ids.push(def.id.clone());
+            }
+        } else {
+            let new_val = amount.min(def.target);
+            let now_complete = new_val >= def.target;
+            let id = Uuid::new_v4().to_string();
+            sqlx::query(
+                "INSERT INTO objective_progress (id, profile_id, objective_id, current, completed, claimed, updated_at) VALUES (?, ?, ?, ?, ?, 0, ?)"
+            )
+            .bind(&id)
+            .bind(profile_id)
+            .bind(&def.id)
+            .bind(new_val)
+            .bind(now_complete)
+            .bind(now)
+            .execute(&mut **tx)
+            .await?;
+            if now_complete {
+                completed_ids.push(def.id.clone());
+            }
+        }
+    }
+
+    Ok(completed_ids)
+}
+
 pub async fn claim_objective(
     pool: &Pool,
     profile_id: &str,