Update database-schema.md (#22)

Author: tqha1011

documentation/architecture/database-schema.md

@@ -2,6 +2,9 @@
 
 The database is built on **PostgreSQL** (hosted via Supabase) and is strictly secured using **Row Level Security (RLS)**.
 
+### Database ERD
+
+<img width="1247" height="620" alt="image" src="https://github.com/user-attachments/assets/30b4850d-744e-4c80-bcc8-d225be763d16" />
 
 
 ### Core Database Dictionary
@@ -16,4 +19,4 @@ The database is built on **PostgreSQL** (hosted via Supabase) and is strictly se
 | **`reminder`** | Scheduling entity for push notifications. | `1:N` with `task`. |
 
 ### Security Architecture
-* **Row Level Security (RLS):** Every core table contains a `profile_id`. RLS policies are enforced at the database level so users can only `SELECT`, `INSERT`, `UPDATE`, or `DELETE` their own data.
+* **Row Level Security (RLS):** Every core table contains a `profile_id`. RLS policies are enforced at the database level so users can only `SELECT`, `INSERT`, `UPDATE`, or `DELETE` their own data.