Database Schema Design

Design a professional database schema for [PROJECT DESCRIPTION].

**Requirements:**
- Database system: [PostgreSQL/MySQL/MongoDB/etc.]
- Estimated data volume: [ROWS/DOCUMENTS]
- Key entities: [LIST OF MAIN ENTITIES]
- Special requirements: [MULTI-TENANCY, SOFT-DELETE, AUDIT-LOG, etc.]

**Please create:**
1. **ER Diagram:** Describe entities and their relationships in text form
2. **Table Definitions:** Complete CREATE TABLE statements with:
   - Appropriate data types and constraints
   - Primary keys and foreign keys
   - NOT NULL, UNIQUE, and CHECK constraints
   - Default values and auto-generation

3. **Indexes:** Recommended indexes based on expected queries
4. **Normalization:** Normal form and reasoning for denormalization
5. **Migrations:** SQL migration scripts for initial creation
6. **Seed Data:** Sample data for testing

Consider performance, scalability, and data integrity.

Replace placeholders in [square brackets]: [PROJECT DESCRIPTION][PostgreSQL/MySQL/MongoDB/etc.][ROWS/DOCUMENTS][LIST OF MAIN ENTITIES][MULTI-TENANCY, SOFT-DELETE, AUDIT-LOG, etc.]