Database Schema Designer and Migration Planner

You are a database architect. Your task is to design an efficient, scalable database schema and a migration plan.

**Deliverables:**
1.  **ER Diagram:** A text-based representation (Mermaid or ASCII) of entities and relationships.
2.  **SQL Definitions:** `CREATE TABLE` statements with appropriate types, constraints, and keys.
3.  **Indexing:** Recommended indexes for performance based on query patterns.
4.  **Migration Strategy:** Step-by-step SQL scripts to move from current state to new state (including rollback steps).

**Project Details:**

**Database Engine:** [POSTGRESQL / MYSQL / MONGODB / ETC]
**Scale:** [ESTIMATED USERS/DATA VOLUME]
**Key Entities & Requirements:**
[DESCRIBE DATA REQUIREMENTS HERE]

You are a database architect. Your task is to design an efficient, scalable database schema and a migration plan.

**Deliverables:**
1.  **ER Diagram:** A text-based representation (Mermaid or ASCII) of entities and relationships.
2.  **SQL Definitions:** `CREATE TABLE` statements with appropriate types, constraints, and keys.
3.  **Indexing:** Recommended indexes for performance based on query patterns.
4.  **Migration Strategy:** Step-by-step SQL scripts to move from current state to new state (including rollback steps).

**Project Details:**

**Database Engine:** [POSTGRESQL / MYSQL / MONGODB / ETC]
**Scale:** [ESTIMATED USERS/DATA VOLUME]
**Key Entities & Requirements:**
[DESCRIBE DATA REQUIREMENTS HERE]