fix: remove multiSchema - follow existing AutoGPT pattern

Remove unnecessary multiSchema configuration that broke existing models.

AutoGPT uses connection string's ?schema=platform parameter as default,
not Prisma's multiSchema feature. Existing models (User, AgentGraph, etc.)
have no @@schema() directives and work fine.

Changes:
- Remove schemas = ["platform", "public"] from datasource
- Remove "multiSchema" from previewFeatures
- Remove all @@schema() directives from LLM models and enum

Migration SQL already creates tables in platform schema explicitly
(CREATE TABLE "platform"."LlmProvider" etc.) which is correct.

This matches the existing pattern used throughout the codebase.

autogpt_platform/backend/schema.prisma

@@ -3,14 +3,13 @@ datasource db {
   url        = env("DATABASE_URL")
   directUrl  = env("DIRECT_URL")
   extensions = [pgvector(map: "vector")]
-  schemas    = ["platform", "public"]
 }
 
 generator client {
   provider               = "prisma-client-py"
   recursive_type_depth   = -1
   interface              = "asyncio"
-  previewFeatures        = ["views", "fullTextSearch", "postgresqlExtensions", "multiSchema"]
+  previewFeatures        = ["views", "fullTextSearch", "postgresqlExtensions"]
   partial_type_generator = "backend/data/partial_types.py"
 }
 
@@ -1343,8 +1342,6 @@ model OAuthRefreshToken {
 enum LlmCostUnit {
   RUN
   TOKENS
-
-  @@schema("platform")
 }
 
 model LlmProvider {
@@ -1364,7 +1361,6 @@ model LlmProvider {
 
   Models LlmModel[]
 
-  @@schema("platform")
 }
 
 model LlmModel {
@@ -1406,7 +1402,6 @@ model LlmModel {
   @@index([providerId, isEnabled])
   @@index([creatorId])
   // Note: slug already has @unique which creates an implicit index
-  @@schema("platform")
 }
 
 model LlmModelCost {
@@ -1429,7 +1424,6 @@ model LlmModelCost {
 
   @@unique([llmModelId, credentialProvider, unit])
   @@index([credentialProvider])
-  @@schema("platform")
 }
 
 model LlmModelCreator {
@@ -1447,7 +1441,6 @@ model LlmModelCreator {
 
   Models LlmModel[]
 
-  @@schema("platform")
 }
 
 model LlmModelMigration {
@@ -1487,5 +1480,4 @@ model LlmModelMigration {
   @@index([targetModelSlug])
   @@index([isReverted])
   @@index([sourceModelSlug, isReverted]) // Composite index for active migration queries
-  @@schema("platform")
 }