fix(db): make purchase_history.ramp_step and purchase_executions.error NOT NULL #1484

Found by the second review of #1473 (#1465).

Problem: two columns are nullable in the schema, but the Go scanners read them into plain values. The Go writers never store NULL, so a NULL can only come from a legacy row or a hand-written SQL row. When one appears, the whole listing fails to scan.

• purchase_history.ramp_step is a nullable INTEGER scanned into a plain int (internal/config/types.go ~1042, store_postgres.go ~2816). Seeding a NULL breaks GetAllPurchaseHistory.
• purchase_executions.error is nullable TEXT. fix(config): tolerate a NULL error when scanning purchase executions #1473 makes the scanner tolerate NULL, but the column can still hold one.

Fix at the root: one migration, numbered above main's highest at merge time, that:

• backfills NULL to 0 for ramp_step and to '' for error;
• sets both columns to NOT NULL DEFAULT.

Include a real-Postgres migration test (up and down) and a check that existing rows survive. Until it lands, scan ramp_step through sql.NullInt32, the same way #1473 handles error.