fix: Refactor token cache management to use in-memory storage and sequential operations

docs/issues/manual_test_auth_fixture_token_refresh_cache_regressions.md

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+---
+title: Manual Test Plan - Auth Fixture Token Refresh/Cache Regressions
+status: Open
+priority: High
+assignee: QA
+labels: testing, auth, regression
+---
+
+## Objective
+
+Validate that recent auth fixture token refresh/cache updates do not introduce login instability, stale session behavior, or parallel test flakiness.
+
+## Preconditions
+
+- Charon test environment is running and reachable.
+- A valid test user account is available.
+- Browser context can be reset between scenarios (clear cookies and site data).
+- Test runner can execute targeted auth fixture scenarios.
+
+## Scenarios
+
+### 1) Baseline Login and Session Reuse
+
+- Step: Sign in once with valid credentials.
+- Step: Run an action that requires authentication.
+- Step: Run a second authenticated action without re-authenticating.
+- Expected outcome:
+  - First action succeeds.
+  - Second action succeeds without unexpected login prompts.
+  - No session-expired message appears.
+
+### 2) Token Refresh Near Expiry
+
+- Step: Start with a session near refresh threshold.
+- Step: Trigger an authenticated action that forces token refresh path.
+- Step: Continue with another authenticated action.
+- Expected outcome:
+  - Refresh occurs without visible interruption.
+  - Follow-up authenticated action succeeds.
+  - No unauthorized or redirect loop behavior occurs.
+
+### 3) Concurrent Authenticated Actions
+
+- Step: Trigger multiple authenticated actions at the same time.
+- Step: Observe completion and authentication state.
+- Expected outcome:
+  - Actions complete without random auth failures.
+  - No intermittent unauthorized responses.
+  - Session remains valid after all actions complete.
+
+### 4) Cache Reuse Across Test Steps
+
+- Step: Complete one authenticated test step.
+- Step: Move to the next step in the same run.
+- Step: Verify auth state continuity.
+- Expected outcome:
+  - Auth state is reused when still valid.
+  - No unnecessary re-login is required.
+  - No stale-token error appears.
+
+### 5) Clean-State Reset Behavior
+
+- Step: Clear session data for a clean run.
+- Step: Trigger an authenticated action.
+- Step: Sign in again when prompted.
+- Expected outcome:
+  - User is correctly prompted to authenticate.
+  - New session works normally after sign-in.
+  - No residual state from previous run affects behavior.
+
+## Bug Capture Template
+
+Use this template for each defect found.
+
+- Title:
+- Date/Time (UTC):
+- Tester:
+- Environment (branch/commit, browser, OS):
+- Scenario ID:
+- Preconditions used:
+- Steps to reproduce:
+  1.
+  2.
+  3.
+- Expected result:
+- Actual result:
+- Frequency (always/intermittent/once):
+- Severity (critical/high/medium/low):
+- Evidence:
+  - Screenshot path:
+  - Video path:
+  - Relevant log snippet:
+- Notes: