diff --git a/lib/db/migrations/005_cross.sql b/lib/db/migrations/005_cross.sql
new file mode 100644
index 0000000..2beaf82
--- /dev/null
+++ b/lib/db/migrations/005_cross.sql
@@ -0,0 +1,43 @@
+CREATE TABLE tags (
+  id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  name        text NOT NULL UNIQUE,
+  description text,
+  color       text,
+  created_at  timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE entity_tags (
+  entity_type text NOT NULL,
+  entity_id   uuid NOT NULL,
+  tag_id      uuid NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+  created_at  timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY (entity_type, entity_id, tag_id)
+);
+
+CREATE TABLE entity_links (
+  id        uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  from_type text NOT NULL,
+  from_id   uuid NOT NULL,
+  to_type   text NOT NULL,
+  to_id     uuid NOT NULL,
+  relation  text NOT NULL DEFAULT 'attached',
+  created_at timestamptz NOT NULL DEFAULT now(),
+  UNIQUE (from_type, from_id, to_type, to_id, relation)
+);
+
+CREATE TABLE attachments (
+  id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  entity_type text,
+  entity_id   uuid,
+  filename    text NOT NULL,
+  mime_type   text,
+  size_bytes  bigint,
+  blob_path   text NOT NULL,
+  checksum    text,
+  uploaded_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_entity_tags_entity ON entity_tags(entity_type, entity_id);
+CREATE INDEX idx_entity_links_from ON entity_links(from_type, from_id);
+CREATE INDEX idx_entity_links_to ON entity_links(to_type, to_id);
+CREATE INDEX idx_attachments_entity ON attachments(entity_type, entity_id);
diff --git a/tests/db/migration_005.test.js b/tests/db/migration_005.test.js
new file mode 100644
index 0000000..d7e328c
--- /dev/null
+++ b/tests/db/migration_005.test.js
@@ -0,0 +1,16 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { resetDb, withClient } from '../helpers/db.js';
+import { migrateUp } from '../../lib/db/migrate.js';
+
+describe('migration 005 — cross', () => {
+  beforeAll(async () => { await resetDb(); await migrateUp(); });
+
+  it('creates tags, entity_tags, entity_links, attachments', async () => {
+    await withClient(async (c) => {
+      for (const t of ['tags','entity_tags','entity_links','attachments']) {
+        const { rows } = await c.query(`SELECT to_regclass('public.' || $1) AS t;`, [t]);
+        expect(rows[0].t).toBe(t);
+      }
+    });
+  });
+});