chore(server): drop unused postgresCredentials feature (#20573)

## Summary

Drops the `postgresCredentials` legacy feature: a never-finished
"postgres proxy" that would have let users query their workspace data
over a standard Postgres connection. Nothing — frontend, e2e, Zapier,
docs, other server code — calls these mutations/query.

## History

- **Introduced** June 2024 (#5767, Thomas Trompette) as "first step for
creating credentials for database proxy", alongside the Postgres FDW /
remote-server work and the custom `twenty-postgres-spilo` image. Planned
follow-ups (provisioning a DB on the proxy, mapping users, exposing it
as a remote server) never landed.
- **Abandoned** January 2026 (#17001, Weiko) when the sibling "remote
integration" feature was removed as a BREAKING CHANGE — "not maintained
for more than a year and never officially launched". The spilo image was
then replaced with vanilla `postgres:16` (#19182, March 2026), retiring
the FDW infrastructure entirely.
- This PR finishes the cleanup: removes the orphaned module, the
`allPostgresCredentials` relation, `JwtTokenTypeEnum.POSTGRES_PROXY` +
payload, the reserved metadata keywords, and adds a 2.5.0 fast instance
command that drops `core.postgresCredentials` (reversible `down`).
Regenerated frontend GraphQL types + SDK metadata client.

## Test plan

- [x] `tsgo --noEmit` clean on twenty-server + twenty-front; lint +
prettier clean on touched files.
- [x] `database:migrate:generate` reports no pending schema diff; server
boots and serves the new schema.

packages/twenty-server/src/engine/core-modules/auth/types/auth-context.type.ts

@@ -43,7 +43,6 @@ export enum JwtTokenTypeEnum {
   LOGIN = 'LOGIN',
   FILE = 'FILE',
   API_KEY = 'API_KEY',
-  POSTGRES_PROXY = 'POSTGRES_PROXY',
   REMOTE_SERVER = 'REMOTE_SERVER',
   KEY_ENCRYPTION_KEY = 'KEY_ENCRYPTION_KEY',
   APPLICATION_ACCESS = 'APPLICATION_ACCESS',
@@ -138,10 +137,6 @@ export type AccessTokenJwtPayload = CommonPropertiesJwtPayload & {
   impersonatedUserWorkspaceId?: string;
 };
 
-export type PostgresProxyTokenJwtPayload = CommonPropertiesJwtPayload & {
-  type: JwtTokenTypeEnum.POSTGRES_PROXY;
-};
-
 export type AppOAuthStateJwtPayload = CommonPropertiesJwtPayload & {
   type: JwtTokenTypeEnum.APP_OAUTH_STATE;
   workspaceId: string;
@@ -170,5 +165,4 @@ export type JwtPayload =
   | RefreshTokenJwtPayload
   | FileTokenJwtPayload
   | FileTokenJwtPayloadLegacy
-  | PostgresProxyTokenJwtPayload
   | AppOAuthStateJwtPayload;