docs: Restructure migration into 6 phases

1. Code - Query syntax, API methods (no DB changes)
2. Settings - Config files (no DB changes)
3. Numeric Types - int->int32, float->float64 (column types)
4. Blob/Attach - Internal blobs (column comments)
5. External Blob/Attach - External storage (column comments)
6. Filepath - Filepath references (column comments)

Each phase independent, testable, and reversible.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko