fix: searchable client combobox in Nova instalação

Replaces the native list with a combobox (name/code/CNPJ search, accent- and
case-insensitive, keyboard + mouse, custom dropdown, hidden client_id = the
Gerenciador's real CLIENTE.ID). The backend resolves client_id against the
active GTI_GERENCIADOR mirror and rejects free text/unknown ids. Adds
Tenant.external_cnpj (migration 0025) mirrored via the internal endpoint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

central/app/models.py

@@ -229,6 +229,7 @@ class Tenant(Base):
     source: Mapped[str] = mapped_column(String(16), default="LOCAL", server_default="LOCAL")  # LOCAL | GTI_GERENCIADOR
     external_client_id: Mapped[int | None] = mapped_column(Integer, nullable=True, index=True)
     external_code: Mapped[str | None] = mapped_column(String(16), nullable=True)
+    external_cnpj: Mapped[str | None] = mapped_column(String(20), nullable=True)  # digits only, for the client selector search
     active: Mapped[bool] = mapped_column(Boolean, default=True, server_default="true")
     synced_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)