refactor: move DB to shared crate, convert push endpoints to server functions, remove dead REST handlers

shared/migrations/002_push_subscriptions.sql

@@ -0,0 +1,13 @@
+-- 002_push_subscriptions.sql
+-- Push notification subscriptions storage
+
+CREATE TABLE IF NOT EXISTS push_subscriptions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    endpoint TEXT NOT NULL UNIQUE,
+    p256dh TEXT NOT NULL,
+    auth TEXT NOT NULL,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Index for faster lookups by endpoint
+CREATE INDEX IF NOT EXISTS idx_push_subscriptions_endpoint ON push_subscriptions(endpoint);