Add database for state management

[gsiener]

Problem

Current implementation is stateless - no memory between runs. This limits:

• Conversation continuity
• Review history
• Multi-document tracking

Proposed Solution

Add a lightweight database (SQLite) to store:

• Document review history
• Conversation threads
• Applied/rejected suggestions
• User preferences

Schema (Draft)

```sql
CREATE TABLE documents (
id TEXT PRIMARY KEY,
title TEXT,
last_reviewed_at TIMESTAMP
);

CREATE TABLE reviews (
id INTEGER PRIMARY KEY,
document_id TEXT,
created_at TIMESTAMP,
prompt TEXT,
response JSON
);

CREATE TABLE conversations (
id INTEGER PRIMARY KEY,
document_id TEXT,
thread_id TEXT,
messages JSON
);
```

Technical Considerations

• SQLite is sufficient for local CLI use
• Consider better-sqlite3 for sync API
• Migration strategy for schema changes