Phase 19: persistent notifications system

New notifications table (migration 0009) stores event-driven alerts
alongside the existing dynamic state notifications (unclaimed objectives,
expiring loans, season ending soon).

Persistent notifications are created automatically during match
processing: one per level gained, one per expired loan card, one per
objective newly completed, and one when a season ends (with
promotion/relegation result and rewards in the body).

GET /notifications now returns a merged list — persistent entries
(newest-first, limit 50) followed by dynamic entries — plus an
unread_count for the badge. Each item carries type, title, body,
is_read, and (for persistent) id and created_at.

PATCH /notifications/:id/read marks a single persistent notification
read. POST /notifications/read-all marks all persistent ones read.

Four new tests: unread_count field, level-up notification creation,
mark-all-read, single-read PATCH. Core now at 77 tests.

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

src/models/notification.rs

@@ -0,0 +1,25 @@
+use serde::Serialize;
+use uuid::Uuid;
+
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct Notification {
+    pub id: String,
+    pub kind: String,
+    pub title: String,
+    pub body: String,
+    pub is_read: bool,
+    pub created_at: String,
+}
+
+impl Notification {
+    pub fn new(kind: &str, title: &str, body: &str) -> Self {
+        Self {
+            id: Uuid::new_v4().to_string(),
+            kind: kind.to_string(),
+            title: title.to_string(),
+            body: body.to_string(),
+            is_read: false,
+            created_at: chrono::Utc::now().to_rfc3339(),
+        }
+    }
+}