feat: add database CHECK constraints for data integrity

Per CodeRabbit feedback - enforce numeric domain rules at DB level:

Migration:
- priceTier: CHECK (priceTier BETWEEN 1 AND 3)
- creditCost: CHECK (creditCost >= 0)
- nodeCount: CHECK (nodeCount >= 0)
- customCreditCost: CHECK (customCreditCost IS NULL OR customCreditCost >= 0)

Schema comments:
- Document constraints inline for developer visibility

Prevents invalid data (negative costs, out-of-range tiers) from
entering the database, matching backend/blocks/llm.py contract.

autogpt_platform/backend/migrations/20260310_add_llm_registry_schema/migration.sql

@@ -138,3 +138,14 @@ ALTER TABLE "platform"."LlmModel" ADD CONSTRAINT "LlmModel_creatorId_fkey" FOREI
 
 -- AddForeignKey
 ALTER TABLE "platform"."LlmModelCost" ADD CONSTRAINT "LlmModelCost_llmModelId_fkey" FOREIGN KEY ("llmModelId") REFERENCES "platform"."LlmModel"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddCheckConstraints (enforce data integrity)
+ALTER TABLE "platform"."LlmModel"
+    ADD CONSTRAINT "LlmModel_priceTier_check" CHECK ("priceTier" BETWEEN 1 AND 3);
+
+ALTER TABLE "platform"."LlmModelCost"
+    ADD CONSTRAINT "LlmModelCost_creditCost_check" CHECK ("creditCost" >= 0);
+
+ALTER TABLE "platform"."LlmModelMigration"
+    ADD CONSTRAINT "LlmModelMigration_nodeCount_check" CHECK ("nodeCount" >= 0),
+    ADD CONSTRAINT "LlmModelMigration_customCreditCost_check" CHECK ("customCreditCost" IS NULL OR "customCreditCost" >= 0);

autogpt_platform/backend/schema.prisma

@@ -1384,7 +1384,7 @@ model LlmModel {
 
   contextWindow   Int
   maxOutputTokens Int?
-  priceTier       Int     @default(1) // 1=cheapest, 2=medium, 3=expensive
+  priceTier       Int     @default(1) // 1=cheapest, 2=medium, 3=expensive (DB constraint: 1-3)
   isEnabled       Boolean @default(true)
   isRecommended   Boolean @default(false)
 
@@ -1412,7 +1412,7 @@ model LlmModelCost {
   updatedAt DateTime    @updatedAt
   unit      LlmCostUnit @default(RUN)
 
-  creditCost Int
+  creditCost Int // DB constraint: >= 0
 
   credentialProvider String
   credentialId       String?
@@ -1460,7 +1460,7 @@ model LlmModelMigration {
   // Track affected nodes as JSON array of node IDs
   // Format: ["node-uuid-1", "node-uuid-2", ...]
   migratedNodeIds Json @default("[]")
-  nodeCount       Int  // Number of nodes migrated
+  nodeCount       Int  // Number of nodes migrated (DB constraint: >= 0)
 
   // Custom pricing override for migrated workflows during the migration period.
   // Use case: When migrating users from an expensive model (e.g., GPT-4) to a cheaper
@@ -1473,7 +1473,7 @@ model LlmModelMigration {
   // If null, the target model's normal cost applies.
   //
   // TODO: Integrate with billing system to apply this override during cost calculation.
-  customCreditCost Int?
+  customCreditCost Int? // DB constraint: >= 0 when not null
 
   // Revert tracking
   isReverted Boolean   @default(false)