Fix posterminal token uniqueness for nullable column

Replace inline UNIQUE constraint with filtered unique index on token
column to allow multiple NULL values while maintaining uniqueness
for non-NULL tokens.

src/schema.js

@@ -577,9 +577,9 @@ BEGIN
         name NVARCHAR(255) NOT NULL,
         token NVARCHAR(255) NULL,
         pairingpin NVARCHAR(50) NOT NULL,
-        CONSTRAINT FK_posterminal_company FOREIGN KEY (company_id) REFERENCES company(id) ON DELETE CASCADE,
-        CONSTRAINT UQ_posterminal_token UNIQUE (token)
+        CONSTRAINT FK_posterminal_company FOREIGN KEY (company_id) REFERENCES company(id) ON DELETE CASCADE
     );
+    CREATE UNIQUE INDEX UQ_posterminal_token ON posterminal(token) WHERE token IS NOT NULL;
 END
 `;