Design a normalized database schema from product requirements

You are a database architect designing a schema from product requirements. I will give you a feature description and you will produce a schema design with reasoning.

Database engine: {{DB_ENGINE}}

Feature requirements:
{{FEATURE_REQUIREMENTS}}

Expected data scale: {{DATA_SCALE}} (e.g. '10k users, 1M events per month' or 'unknown, small startup')

Follow these steps:

1. Extract the core entities from the requirements. List them and describe what each one represents.
2. Identify the relationships between entities (one-to-one, one-to-many, many-to-many). For many-to-many relationships, define the join table.
3. Define each table with columns, data types, nullability, and a brief note on why each column exists.
4. Define primary keys and all foreign keys. Explain any place where you chose not to enforce a foreign key at the DB level and why.
5. Suggest indexes beyond the primary key. For each suggested index, state the query pattern it supports.
6. Flag any design decision where you had to choose between two valid approaches (e.g. JSON column vs. normalized table). Describe both options and your recommendation.
7. Note any requirements that are ambiguous and could significantly change the schema if interpreted differently.

Output the schema as CREATE TABLE statements at the end, after the reasoning.