chore(db): drop users.salt once no running instance reads it #1956

Follow-up split out of #313 (everything else in #313 is done on main).

users.salt is no longer read or written by any non-test Go code (#1514 removed the reads, #1560 unified the column list). What is left:

• A migration ALTER TABLE users DROP COLUMN salt.
• The raw-SQL test fixtures that still INSERT salt (about 15 across internal/api/*_integration_test.go and other fixtures) must change in the same commit as the migration.

Deploy gate: migrations run at server startup, so the drop must not ship until no running instance reads the column (a rolling deploy would break logins on old instances). Land it only after the #1514 build is deployed everywhere.