Remove model from promptVariant and add cost

Storing the model on promptVariant is problematic because it isn't always in sync with the actual prompt definition. I'm removing it for now to see if we can get away with that -- might have to add it back in later if this causes trouble.

Added `cost` to modelOutput as well so we can cache that, which is important given that the cost calculations won't be the same between different API providers.

prisma/schema.prisma

@@ -94,7 +94,7 @@ model ScenarioVariantCell {
     streamingChannel String?
     retrievalStatus  CellRetrievalStatus @default(COMPLETE)
 
-    modelOutput      ModelOutput?
+    modelOutput ModelOutput?
 
     promptVariantId String        @db.Uuid
     promptVariant   PromptVariant @relation(fields: [promptVariantId], references: [id], onDelete: Cascade)
@@ -114,6 +114,7 @@ model ModelOutput {
     inputHash        String
     output           Json
     timeToComplete   Int    @default(0)
+    cost             Float?
     promptTokens     Int?
     completionTokens Int?