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/database/commands/upgrade-version-command/instance-commands.constant.ts

@@ -39,6 +39,7 @@ import { EncryptApplicationRegistrationVariableSlowInstanceCommand } from 'src/d
 import { EncryptSigningKeyPrivateKeysSlowInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-slow-1798000007000-encrypt-signing-key-private-keys';
 import { EncryptSensitiveConfigStorageSlowInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-slow-1798000008000-encrypt-sensitive-config-storage';
 import { AddSubFieldNameToViewSortFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-fast-1778502963794-add-sub-field-name-to-view-sort';
+import { DropPostgresCredentialsTableFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-fast-1798500000000-drop-postgres-credentials-table';
 
 export const INSTANCE_COMMANDS = [
   AddViewFieldGroupIdIndexOnViewFieldFastInstanceCommand,
@@ -80,4 +81,5 @@ export const INSTANCE_COMMANDS = [
   EncryptSigningKeyPrivateKeysSlowInstanceCommand,
   EncryptSensitiveConfigStorageSlowInstanceCommand,
   AddSubFieldNameToViewSortFastInstanceCommand,
+  DropPostgresCredentialsTableFastInstanceCommand,
 ];