feat(er): added ER diagram

src/main/resources/db/ER.md

@@ -0,0 +1,27 @@
+# ER
+
+```mermaid
+erDiagram
+    users {
+        UUID id PK
+        VARCHAR name
+        VARCHAR email
+    }
+
+    films {
+        UUID id PK
+        VARCHAR title
+        TEXT description
+    }
+
+    favorites {
+        UUID id PK
+        UUID user_id FK
+        UUID film_id FK
+        VARCHAR comment
+        BOOLEAN is_viewed
+    }
+
+    users ||--o{ favorites : has
+    films ||--o{ favorites : linked
+```