fix: auto-recover corrupt LanceDB tables on startup

[DevNexsler]

Summary

• auto-recover unreadable LanceDB tables during MCP lazy init and indexer startup
• share a targeted open-with-recovery path instead of failing file_status / first index open on known Lance corruption signatures
• add regression coverage for recovery open path and MCP store bootstrap wiring

Evidence

• doc-organizer session logs in last 24h showed repeated MCP failures around 2026-05-19 09:03-09:30 America/New_York:
  • service_unavailable: LanceError(IO): Generic memory error: Invalid range 0..0 for object of size 0 bytes
  • retrieval_failed: missing chunks.lance manifest
• container recovered only after restart/reindex, which means init-time corruption handling was incomplete

Root Cause

flow_index_vault had post-index recovery, but startup paths still opened LanceDB directly. If table was unreadable before indexing began, file_status and first index open failed before recovery code ran.

Fix

• add open_store_with_recovery() and recover_corrupt_table() in lancedb_store.py
• use recovery-aware open in mcp_server._build_store_and_embed()
• use recovery-aware open in flow_index_vault.index_vault_flow()

Test Report

• PYTHONPATH=. pytest -q tests/test_store.py -k 'open_store_with_recovery'
• PYTHONPATH=. pytest -q tests/test_mcp_contract.py -k 'build_store_and_embed_uses_recovery_open or get_deps_failure_in_status'
• PYTHONPATH=. pytest tests/test_config.py tests/test_prefect_server.py

Required suite output:

============================= test session starts ==============================
platform linux -- Python 3.12.3, pytest-9.0.2, pluggy-1.6.0
rootdir: /tmp/ragbox-health-fix
configfile: pyproject.toml
plugins: asyncio-1.3.0, anyio-4.13.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 24 items

tests/test_config.py ............                                        [ 50%]
tests/test_prefect_server.py ............                                [100%]

============================== 24 passed in 1.23s ==============================