Phase 20: achievement system

18 data-driven achievements (achievements.json) across 8 trigger categories:
matches_played, matches_won, goals_scored, packs_opened, sbcs_completed,
cards_owned, level, objectives_completed, drafts_completed. Rarities span
common → epic. Coin rewards range from 500 (first_match) to 6000 (win_50).

check_and_unlock() queries the relevant metric from existing tables, skips
already-earned achievements via INSERT OR IGNORE, grants coin rewards, and
fires a persistent notification per unlock. Trigger values are cached per
call to avoid redundant DB round-trips for same-trigger achievements.

Checks run automatically after every match result (all triggers), every
pack open (packs_opened), and every successful SBC submission (sbcs_completed).

GET /achievements returns all definitions annotated with unlocked/unlocked_at,
plus earned and total counts. POST /matches/result response gains an
achievements_unlocked array (empty when nothing new unlocked).

AppState gains achievement_defs (Arc<Vec<AchievementDefinition>>) loaded
from data/achievements/**/*.json at startup — same pattern as obj_defs.

5 new tests: list endpoint, first_match unlock, first_win unlock, no-dup
guard, coin reward verification. Core now at 82 tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/models/achievement.rs

@@ -0,0 +1,20 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AchievementDefinition {
+    pub id: String,
+    pub title: String,
+    pub description: String,
+    pub icon: String,
+    pub trigger: String,
+    pub threshold: i64,
+    pub reward_coins: i64,
+    pub rarity: String,
+}
+
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct PlayerAchievement {
+    pub id: String,
+    pub achievement_id: String,
+    pub unlocked_at: String,
+}