fix: expose mock exam content readiness

[vinicius-ssantos]

Fixes the production path where a fresh CertForge installation offered a mock-exam action that could only fail with insufficient_mock_content.

What changes:

• adds GET /api/study/mock-exams/availability;
• reports readiness from the same per-topic eligibility rule used by the planner;
• reports missing reviewed/published question counts per topic;
• keeps an already-active mock resumable even if the currently-published bank later falls below the blueprint;
• updates the learner track page, EN/PT-BR copy, OpenAPI contract/types, tests and changelog.

Validation completed locally:

• full mvn verify on Java 25: 219 unit tests + 183 integration tests, 0 failures; Spotless and PMD passed
• frontend: 181 tests passed
• frontend typecheck/lint/API contract/build passed (one pre-existing hooks warning only)
• git diff --check clean

Closes #138

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-07T17:11:54.400344Z	a1a92de	PR opened

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Reviewed commit: a1a92de854

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[chatgpt-codex-connector]

Refresh availability after mock lifecycle changes

This learner-specific query inherits the 30-second staleTime from queryClient.ts, but neither starting nor finishing a mock invalidates or updates it. If a learner starts a mock and returns with browser Back during that window, the cached activeSessionId: null still renders “Start” rather than “Continue”; conversely, after continuing and quickly finishing an existing mock, the cached session ID sends them back to the closed run instead of offering a new mock. Update or invalidate ['mock-exam-availability', slug] when the lifecycle changes, or force this query to refetch on remount.

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