[Requires "warm" cache flush (no immediate downtime before flush)] Migrate viewGroup.fieldMetadataId -> view.mainGroupByFieldMetadataId (1/3) (#16206)

In this PR (1/3)
- introduce view.mainGroupByFieldMetadataId as the new reference
determining which fieldMetadataId is used in a grouped view, in order to
deprecate viewGroup.fieldMetadataId which creates inconsistencies.
view.mainGroupByFieldMetadataId is now filled at every view creation,
though not in use yet.
- Introduce a command to backfill view.mainGroupByFieldMetadataId for
existing views + delete all viewGroup.fieldMetadataId with a
fieldMetadataId that is not view.mainGroupByFieldMetadataId. (It should
concern 37 active workspaces)
- Temporarily disable the option to change a grouped view's
fieldMetadataId as for now it creates inconsistencies. This feature can
be reintroduced when we have done the full migration.

In a next PR
- (2/3) use view.mainGroupByFieldMetadataId instead of
viewGroup.fieldMetadataId. In FE we may keep viewGroup.fieldMetadataId
as a state (TBD). View groups will now be created / deleted as a side
effect of view's mainGroupByFieldMetadataId update.
- (3/3) remove viewGroup.fieldMetadataId

---------

Co-authored-by: Charles Bochet <charles@twenty.com>

.cursor/rules/README.mdc

@@ -12,6 +12,7 @@ This directory contains Twenty's development guidelines and best practices in th
 ### Core Guidelines
 - **architecture.mdc** - Project overview, technology stack, and infrastructure setup (Always Applied)
 - **nx-rules.mdc** - Nx workspace guidelines and best practices (Auto-attached to Nx files)
+- **server-migrations.mdc** - Backend migration and TypeORM guidelines for `twenty-server` (Auto-attached to server entities and migration files)
 
 ### Code Quality
 - **typescript-guidelines.mdc** - TypeScript best practices and conventions (Auto-attached to .ts/.tsx files)
@@ -40,7 +41,7 @@ You can manually reference any rule using the `@ruleName` syntax:
 - `@testing-guidelines` - Get testing recommendations
 
 ### Rule Types Used
-- **Always Applied** - Loaded in every context (architecture.mdc, README.mdc)  
+- **Always Applied** - Loaded in every context (architecture.mdc, README.mdc)
 - **Auto Attached** - Loaded when matching file patterns are referenced
 - **Agent Requested** - Available for AI to include when relevant
 - **Manual** - Only included when explicitly mentioned

.cursor/rules/server-migrations.mdc

@@ -0,0 +1,29 @@
+---
+description: Guidelines for generating and managing TypeORM migrations in twenty-server
+globs: [
+  "packages/twenty-server/src/**/*.entity.ts",
+  "packages/twenty-server/src/database/typeorm/**/*.ts"
+]
+alwaysApply: false
+---
+
+## Server Migrations (twenty-server)
+
+- **When changing an entity, always generate a migration**
+  - If you modify a `*.entity.ts` file in `packages/twenty-server/src`, you **must** generate a corresponding TypeORM migration instead of manually editing the database schema.
+  - Use the Nx + TypeORM command from the project root:
+
+    ```bash
+    npx nx run twenty-server:typeorm migration:generate src/database/typeorm/core/migrations/common/[name] -d src/database/typeorm/core/core.datasource.ts
+    ```
+
+  - Replace `[name]` with a descriptive, kebab-case migration name that reflects the change (for example, `add-agent-turn-evaluation`).
+
+- **Prefer generated migrations over manual edits**
+  - Let TypeORM infer schema changes from the updated entities; only adjust the generated migration file manually if absolutely necessary (for example, for data backfills or complex constraints).
+  - Keep schema changes (DDL) in these generated migrations and avoid mixing in heavy data migrations unless there is a strong reason and clear comments.
+
+- **Keep migrations consistent and reversible**
+  - Ensure the generated migration includes both `up` and `down` logic that correctly applies and reverts the entity change when possible.
+  - Do not delete or rewrite existing, committed migrations unless you are explicitly working on a pre-release branch where history rewrites are allowed by team conventions.
+