Document account status enum values and valid transitions

[phertyameen]

Location

docs/database-schema.md

Problem

With INITIALIZING, CLAIMING, PARTIAL_SWEEP, and others added incrementally via separate migrations, no confirmed single reference documents the full current enum and its valid state transitions for someone new to the codebase.

Acceptance Criteria

• Every current account_status value listed with a plain-language description
• Valid transition diagram or table included, matching the state-machine implementation from section B
• Kept in sync going forward as a documented responsibility when new statuses are added