fix(ai) - add ai model preferences fallback (#20704)

**Problem** 
AI_MODEL_PREFERENCES, JSON env var is not supported +
IS_CONFIG_VARIABLES_IN_DB_ENABLED=false in twenty cloud server
-> No option to set AI_MODEL_PREFERENCES

**Solution** 
AI_MODEL_PREFERENCES supports three override sources beyond the
hardcoded code defaults, in priority order:

- DB (IS_CONFIG_VARIABLES_IN_DB_ENABLED=true), the only writable source;
admin-panel mutations persist here
- ENV not usable in Twenty Cloud, which does not handle JSON-format env
vars
- **Introduced in this PR** --> File
(AI_MODEL_PREFERENCES_STORAGE_PATH), a read-only startup fallback, the
only viable override in Cloud/self-managed deployments where DB config
is disabled and JSON env vars are unsupported.

packages/twenty-server/src/engine/core-modules/twenty-config/config-variables.ts

@@ -44,7 +44,7 @@ import {
 } from 'src/engine/core-modules/twenty-config/twenty-config.exception';
 import { type AiModelPreferences } from 'src/engine/metadata-modules/ai/ai-models/types/ai-model-preferences.type';
 import { type AiProvidersConfig } from 'src/engine/metadata-modules/ai/ai-models/types/ai-providers-config.type';
-import { loadDefaultModelPreferences } from 'src/engine/metadata-modules/ai/ai-models/utils/load-default-model-preferences.util';
+import { DEFAULT_MODEL_PREFERENCES } from 'src/engine/metadata-modules/ai/ai-models/utils/load-default-model-preferences.util';
 
 export class ConfigVariables {
   @ConfigVariablesMetadata({
@@ -1429,7 +1429,16 @@ export class ConfigVariables {
     type: ConfigVariableType.JSON,
   })
   @IsOptional()
-  AI_MODEL_PREFERENCES: AiModelPreferences = loadDefaultModelPreferences();
+  AI_MODEL_PREFERENCES: AiModelPreferences = DEFAULT_MODEL_PREFERENCES;
+
+  @ConfigVariablesMetadata({
+    group: ConfigVariablesGroup.LLM,
+    description:
+      'Storage path for AI model preferences fallback (e.g. config/ai-model-preferences.json). Loaded at startup and used only when no value is set via AI_MODEL_PREFERENCES env var or the database.',
+    type: ConfigVariableType.STRING,
+  })
+  @IsOptional()
+  AI_MODEL_PREFERENCES_STORAGE_PATH?: string;
 
   @ConfigVariablesMetadata({
     group: ConfigVariablesGroup.SERVER_CONFIG,