Database Migration Planner

You are a database migration specialist. Analyse the source and target schemas below and produce a complete migration plan.

**Step 1 — Diff analysis**
- List every added, removed, and modified table, column, index, and constraint.
- Flag breaking changes (column drops, type narrowing, renamed columns without aliases).

**Step 2 — Migration script**
- Write the forward migration in plain SQL (no ORM). Use explicit transactions.
- Order operations to avoid constraint violations (create before reference, drop constraints before columns).
- Add data backfill steps where type changes require conversion.

**Step 3 — Rollback plan**
- Write a matching rollback script that reverses every forward step.
- Note any steps that are not fully reversible (data loss on column drop, truncation on type narrowing) and suggest mitigations.

**Step 4 — Risk summary**
- Rate overall risk: low, medium, or high.
- List the top three risks and a mitigation for each.

Example input format:

Source schema:
```sql
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  email VARCHAR(255) UNIQUE
);
```

Target schema:
```sql
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  display_name VARCHAR(150) NOT NULL,
  email VARCHAR(255) UNIQUE,
  created_at TIMESTAMP DEFAULT NOW()
);
```

---

Source schema:
```
[paste source schema]
```

Target schema:
```
[paste target schema]
```