Preview boots fail on their own schema migration ("table already exists"): 11 of 12 failed deploys #558

Study 2026-10-05: 11 of the 12 failed preview deploys since 10-01 died at boot on the schema migration with "table already exists", and 7 pulls were closed because their preview never came up. A preview forge starts from copied data; a pull that adds a table (or a second boot of the same preview) re-runs a CREATE that is not idempotent.

Acceptance:

• [ ] reproduce in a test: boot a preview on copied data, boot it again, and boot a branch that adds a table on data that already has it
• [ ] migrations are idempotent or tracked (a schema_version / applied-migrations table); a second boot is a no-op
• [ ] a preview whose migration fails says which statement failed on the pull, in one line
• [ ] remove the empty 639 KB forge.db that #433 committed to main and ignore *.db at the repo root

Mason fits.