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-front/src/generated-metadata/graphql.ts

@@ -2455,11 +2455,9 @@ export type Mutation = {
   destroyViewFilterGroup: Scalars['Boolean'];
   destroyViewGroup: ViewGroup;
   destroyViewSort: Scalars['Boolean'];
-  disablePostgresProxy: PostgresCredentials;
   duplicateDashboard: DuplicatedDashboard;
   editSSOIdentityProvider: EditSso;
   emailPasswordResetLink: EmailPasswordResetLink;
-  enablePostgresProxy: PostgresCredentials;
   endSubscriptionTrialPeriod: BillingEndTrialPeriod;
   evaluateAgentTurn: AgentTurnEvaluation;
   executeOneLogicFunction: LogicFunctionExecutionResult;
@@ -4030,14 +4028,6 @@ export type PlaceDetailsResult = {
   street?: Maybe<Scalars['String']>;
 };
 
-export type PostgresCredentials = {
-  __typename?: 'PostgresCredentials';
-  id: Scalars['UUID'];
-  password: Scalars['String'];
-  user: Scalars['String'];
-  workspaceId: Scalars['UUID'];
-};
-
 export type PublicApplicationRegistration = {
   __typename?: 'PublicApplicationRegistration';
   id: Scalars['UUID'];
@@ -4161,7 +4151,6 @@ export type Query = {
   getPageLayoutWidget: PageLayoutWidget;
   getPageLayoutWidgets: Array<PageLayoutWidget>;
   getPageLayouts: Array<PageLayout>;
-  getPostgresCredentials?: Maybe<PostgresCredentials>;
   getPublicWorkspaceDataByDomain: PublicWorkspaceData;
   getPublicWorkspaceDataById: PublicWorkspaceDataSummary;
   getResourceCreditUsage: Array<BillingResourceCreditUsage>;