chore(server): rename TypeORM migrations dir to legacy-typeorm-migrations-do-not-add (#21295)

## What

Renames the historical TypeORM migrations directory so it's obvious at a
glance the path is frozen.

- `packages/twenty-server/src/database/typeorm/core/migrations/common/`
→ `…/typeorm/core/legacy-typeorm-migrations-do-not-add/common/`
- `packages/twenty-server/src/database/typeorm/core/migrations/billing/`
→ `…/typeorm/core/legacy-typeorm-migrations-do-not-add/billing/`
- `packages/twenty-server/src/database/typeorm/core/migrations/utils/` —
**left in place** (those SQL helpers are still imported by current
instance/workspace commands, see e.g.
`1-21-workspace-command-1775500002000-add-global-key-value-pair-unique-index.command.ts`)
- Updates `core.datasource.ts` globs to the new path and adds an inline
comment explaining the dir is frozen
- Adds a `README.md` at the new dir pointing readers at
`UPGRADE_COMMANDS.md` and the active `upgrade-version-command/` tree

## Why

The TypeORM migration system was replaced by fast/slow instance commands
+ workspace commands (PR #19356), but `typeorm/core/migrations/common/`
still looked structurally identical to an active migrations folder, with
new files merging in as recently as last week. New contributors — and AI
agents — kept inferring it was the active path and adding TypeORM
`MigrationInterface` files. See #21286 for the most recent instance.

This is recommendation **1b** from #21286
(`https://github.com/twentyhq/twenty/pull/21286#discussion_r3369356792`).
A renamed folder is the single strongest signal — no one instinctively
adds to a `do-not-add` directory.

## Notes

- Imports from `src/database/typeorm/core/migrations/utils/…` keep
working because `utils/` did not move.
- No behavior change at runtime: TypeORM loads the same files via
`_typeorm_migrations`, just from the new path.

## Test plan

- [ ] CI green
- [ ] `nx build twenty-server` succeeds
- [ ] Fresh `nx database:reset twenty-server` from a clean DB still
replays the legacy TypeORM migrations (i.e. `_typeorm_migrations` rows
get inserted at boot/init)
- [ ] Spot-check that an instance command that imports from
`migrations/utils/` still resolves at build time (e.g.
`1-21-workspace-command-1775500002000-add-global-key-value-pair-unique-index.command.ts`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Félix Malfait <FelixMalfait@users.noreply.github.com>

packages/twenty-server/src/database/typeorm/core/core.datasource.ts

@@ -56,14 +56,19 @@ export const typeORMCoreModuleOptions: TypeOrmModuleOptions = {
   migrationsRun: false,
   migrationsTableName: '_typeorm_migrations',
   metadataTableName: '_typeorm_generated_columns_and_materialized_views',
+  // The TypeORM migration system is frozen — historical migrations live in
+  // `legacy-typeorm-migrations-do-not-add/` and are loaded here only so the
+  // `_typeorm_migrations` table stays consistent for older deployments.
+  // Do NOT add new files there: write a fast/slow instance command instead.
+  // See `packages/twenty-server/docs/UPGRADE_COMMANDS.md`.
   migrations:
     process.env.IS_BILLING_ENABLED === 'true'
       ? [
-          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/migrations/common/*{.ts,.js}`,
-          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/migrations/billing/*{.ts,.js}`,
+          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/legacy-typeorm-migrations-do-not-add/common/*{.ts,.js}`,
+          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/legacy-typeorm-migrations-do-not-add/billing/*{.ts,.js}`,
         ]
       : [
-          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/migrations/common/*{.ts,.js}`,
+          `${isJest ? 'src/' : 'dist/'}database/typeorm/core/legacy-typeorm-migrations-do-not-add/common/*{.ts,.js}`,
         ],
   ssl:
     process.env.PG_SSL_ALLOW_SELF_SIGNED === 'true'

packages/twenty-server/src/database/typeorm/core/legacy-typeorm-migrations-do-not-add/README.md

@@ -0,0 +1,23 @@
+# Legacy TypeORM migrations — do not add new files here
+
+This directory contains historical TypeORM migrations (`common/` and `billing/`).
+They are kept so that older deployments can still replay them against the
+`_typeorm_migrations` table.
+
+**The TypeORM migration system is frozen.** Do not add new files here.
+
+The active upgrade system is **instance commands** (fast / slow) and
+**workspace commands**, registered with `@RegisteredInstanceCommand` and
+`@RegisteredWorkspaceCommand`. Generate one with:
+
+```bash
+npx nx run twenty-server:database:migrate:generate --name <name> --type <fast|slow>
+```
+
+See `packages/twenty-server/docs/UPGRADE_COMMANDS.md` for the full guide and
+`packages/twenty-server/src/database/commands/upgrade-version-command/` for
+existing examples.
+
+Note: `../migrations/utils/` is **not** legacy — those SQL helpers are still
+imported by current instance/workspace commands and live outside this folder
+on purpose.