Document the database schema in DATABASE.md

Every table, column, foreign key and unique index (checked against a database built
from the migrations), the JSON stored in text columns, the settings keys, delete
behaviour, retention and backups, and how to change the schema. Linked from the
README alongside the other documents.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -327,6 +327,11 @@ needs the site to be served over HTTPS (browsers only offer install on secure
 origins; `localhost` also counts). The bundled service worker deliberately
 caches nothing, so an installed copy always shows the current build.
 
+**More documentation**: [ROLES.md](ROLES.md) — who can see and do what;
+[NOTIFICATIONS.md](NOTIFICATIONS.md) — every notification the app sends and when;
+[INTEGRATIONS.md](INTEGRATIONS.md) — the credentials each integration needs;
+[DATABASE.md](DATABASE.md) — the database tables, columns and relationships.
+
 ## Requirements
 
 - Node.js 20+