docs: merge notification system docs into single Native-First guide

- Consolidate 5 notification-system-* files into doc/notification-system.md
- Add web-push cleanup guide and Start-on-Login glossary entry
- Configure markdownlint for consistent formatting
- Remove web-push references, focus on native OS scheduling

Reduces maintenance overhead while preserving all essential information
in a single, well-formatted reference document.

src/db-sql/migration.ts

@@ -144,6 +144,12 @@ const MIGRATIONS = [
       INSERT INTO active_identity (id, activeDid, lastUpdated) VALUES (1, NULL, datetime('now'));
       `,
   },
+  {
+    name: "003_add_hasBackedUpSeed_to_settings",
+    sql: `
+      ALTER TABLE settings ADD COLUMN hasBackedUpSeed BOOLEAN DEFAULT FALSE;
+    `,
+  },
 ];
 
 /**