Database Schema Designer

Design a database schema for the following system:

**System**: {{system}}
**Requirements**: {{requirements}}
**Database**: {{database}} (PostgreSQL/MySQL)
**Expected scale**: {{scale}} (rows, queries/sec)

Provide:
1. **Entity-relationship description**: entities, relationships (1:1, 1:N, N:M), cardinality
2. **Table definitions**: CREATE TABLE statements with:
   - Primary keys (UUID vs serial)
   - Foreign keys with ON DELETE behavior
   - NOT NULL constraints
   - CHECK constraints for data validation
   - DEFAULT values
   - Indexes (B-tree, GIN, partial) with justification
3. **Normalization**: current normal form, any intentional denormalization with reasoning
4. **Migration script**: in order, idempotent (IF NOT EXISTS)
5. **Sample queries**: the 5 most common queries and how the schema supports them
6. **Scaling considerations**: partitioning strategy, read replica compatibility