Allow clear cache sessions

[benidk]

Description

Following this issue

Administrators had no way to invalidate active authentication-approval bypasses (the cached "remembered web approval" that lets a login skip re-approval for the configured grace period) without waiting it out. This adds that capability, for use during offboarding, a suspected key compromise, or a policy change.

Backend (warpgate-admin, warpgate-core)

• GET /web-approvals — list currently active (unexpired) cached bypasses,
  aggregated across the cluster.
• DELETE /web-approvals — clear every cached bypass (global).
• DELETE /web-approvals/:username — clear all cached bypasses for one user.
• DELETE /web-approvals/:username/scope — clear a single target's (or,
  via all_targets, every target's) bypass for one user, for row-level
  revoke from the UI.

All mutating endpoints require the config_edit admin permission. The
cache is per-node in-memory, so clears fan out to every other cluster node
(same pattern as session termination), and reads aggregate results from
all nodes too.

Web Admin UI

New "Web approvals" page under Status: lists active bypasses (user,
protocol, target/scope, source IP, granted-at), with:

• a "Clear all" button,
• a dropdown of users currently holding a cached bypass, to clear all of
  that user's bypasses,
• a per-row "Revoke" action that clears only that row's specific
  target/scope rather than the whole user.

Tests

• Unit tests for the new AuthStateStore / Services methods
  (clear all / clear for user / clear for user+scope / list active).
• Added the missing AdminApiTestCase entries for the 4 new operations so
  the admin-API permission-coverage test passes.

AI Usage

Choose the level of AI involvement for this PR.

• Fully vibe coded
• AI-designed, AI-coded, manually checked
• Human-designed, AI-coded
• Human-designed, human-coded (includes AI autocompletions and boilerplate gen)

_{This is not to block AI contributions but rather to speed up PR review (saves time on trying to deduce the logic behind AI hallucinations).}