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>

migrations/0009_notifications.sql

@@ -0,0 +1,13 @@
+-- Persistent event-driven notifications (level-ups, loan expiry, season end, etc.)
+-- Dynamic state notifications (expiring loans, unclaimed objectives) remain in code.
+CREATE TABLE IF NOT EXISTS notifications (
+    id          TEXT    PRIMARY KEY,
+    kind        TEXT    NOT NULL,
+    title       TEXT    NOT NULL,
+    body        TEXT    NOT NULL,
+    is_read     INTEGER NOT NULL DEFAULT 0,
+    created_at  TEXT    NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_notifications_is_read ON notifications (is_read);
+CREATE INDEX IF NOT EXISTS idx_notifications_created  ON notifications (created_at DESC);