Fix schema init crash on existing DBs with old verification_codes table

Move auth-related indexes into migrations so they run after column
additions. Previously CREATE INDEX on verification_codes(identifier)
would fail because the old table still had the email column.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrate.go

@@ -15,6 +15,9 @@ func runMigrations(database *sql.DB) {
 		`ALTER TABLE users ADD COLUMN email TEXT UNIQUE`,
 		// Verification codes use a generic identifier column.
 		`ALTER TABLE verification_codes ADD COLUMN identifier TEXT NOT NULL DEFAULT ''`,
+		// Indexes for auth tables (created here so they run after column migrations).
+		`CREATE INDEX IF NOT EXISTS idx_sessions_user ON sessions(user_id)`,
+		`CREATE INDEX IF NOT EXISTS idx_verification_codes_identifier ON verification_codes(identifier)`,
 	}
 	for _, m := range migrations {
 		_, err := database.Exec(m)