Sanitize dashboard/status to prevent leaking brief contents

[ianalloway]

Motivation

• The live dashboard and status SSE endpoints exposed full Markdown report contents by reading data/reports/*.md and embedding them into the status payload and rendered JS, bypassing the HMAC-protected delivery route semantics.
• Any unauthenticated remote client reachable to the server could retrieve confidential briefs without a delivery token, so status/event endpoints must not include report bodies.

Description

• Stop reading and embedding report Markdown in build_status_data() so it only enumerates brief IDs; report bodies are no longer loaded into the briefs map in solvent/dashboard.py.
• Add a _sanitize_status_data() wrapper used by _refresh_status() and external status-file polling in solvent/server.py to redact briefs values (expose only IDs with empty values) before HTTP/SSE publishing.
• Require delivery token checks on the preview API GET /api/briefs/{job_id} (added token parameter and verify_delivery_token guard) to match the canonical hosted brief route semantics in solvent/server.py.
• Change dashboard client JS (rendered by solvent/dashboard.py) to avoid rendering embedded brief text and to show a placeholder message directing users to the token-protected hosted brief link.
• Add regression tests in tests/test_server.py to assert status/dashboard pages do not disclose brief contents and that the preview API enforces delivery tokens.

Testing

• Compiled changed modules with python -m py_compile solvent/dashboard.py solvent/server.py tests/test_server.py which succeeded.
• Ran python -m unittest tests.test_server; tests executed in this environment but were skipped where the fastapi test client was not available (appropriate skip behavior), so headless client checks were not executed here.
• Executed a targeted sentinel check (small Python run) that created a report file and validated that build_status_data() and render() do not include the secret; this check passed.
• Ran full python -m unittest discover; this run surfaced unrelated environment/test-state failures (missing optional rich dependency causing skipped tests and an unrelated test observing persisted jobs), indicating the failures are not caused by the dashboard redaction changes.

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