Split identity server admin into focused modules

Refactor `crates/identity-server/src/admin.rs` from a single large file into a focused Rust module tree while preserving the existing admin routes and behavior. Constraints: - Keep the `admin::routes()` integration point unchanged. - Keep HTTP behavior, localization, CSRF/session handling, and Askama rendering unchanged. - Reuse shared helpers and view models rather than duplicating them across feature modules. - Do not alter the supported API surface or add new admin behavior. Acceptance criteria: - Admin handlers are grouped into coherent submodules with clear shared boundaries. - The original monolithic file is replaced by a concise module root. - Formatting, clippy, workspace tests, and the admin contract tests pass.