refactor(users): rename Prisma User model to UserDirectoryEntry

Mechanical refactor, prerequisite to ADR-0026 PR 1. Frees the
`User` name for the upcoming ADR-0026 model (UUID PK + FK to
Person + lastSignInAt), which has materially different semantics
from the ADR-0020 user-directory cache.

Zero behavioural change. Same columns, same indexes, same
constraints, same call sites - just a different identifier on the
model, the table, and the corresponding Prisma client field.

Two renames in user-directory.service.ts: the Prisma model rename
collided with an existing TS interface also called
UserDirectoryEntry (which was actually the input shape of
recordSignIn). The interface gets renamed to RecordSignInInput at
the same time, which is its correct semantic name. Net effect:
clearer on both sides.

Postgres ALTER TABLE RENAME does NOT cascade to PK constraint /
index names; the migration explicitly ALTER INDEXes the three
named indexes (pkey + last_seen_at + username).

The class name AdminUsersReader, the endpoint URL /api/admin/users,
the DTO AdminUserDto and the local interface UserRow are
unchanged - these are SPA/HTTP-facing identifiers where the
"admin user list" URL semantics hold regardless of the backing
table name.

apps/portal-bff/prisma/migrations/20260526200000_rename_users_to_user_directory_entries/migration.sql

@@ -0,0 +1,19 @@
+-- Rename `users` -> `user_directory_entries`.
+--
+-- Prepares for ADR-0026 PR 1 which introduces a NEW `User` model
+-- (UUID PK + FK to `Person` + `lastSignInAt`) with materially
+-- different semantics. The existing model (ADR-0020 sign-in cache,
+-- Entra `oid` as PK) keeps its role but gets a more precise name
+-- that won't collide.
+--
+-- Mechanical refactor only — same columns, same indexes, same
+-- constraints. ADR-0026 PR 1 lands the new `users` table with
+-- the new semantics.
+
+ALTER TABLE "users" RENAME TO "user_directory_entries";
+
+-- Rename the PK constraint + indexes so they track the new table name.
+-- Postgres doesn't auto-rename these on ALTER TABLE RENAME.
+ALTER INDEX "users_pkey" RENAME TO "user_directory_entries_pkey";
+ALTER INDEX "users_last_seen_at_idx" RENAME TO "user_directory_entries_last_seen_at_idx";
+ALTER INDEX "users_username_idx" RENAME TO "user_directory_entries_username_idx";