chore: backend quick-win dead-code cleanup

[jlunder00]

Summary

• Remove dead get_orchestrator_conversation import from bot/message_handler.py (import-only, no callers)
• Remove first (duplicate) _OAUTH_TOKEN_RE = re.compile(...) in api/routes/integrations.py — both definitions are identical; the second at line 79 shadows the first at line 75
• Remove search_tasks, journal (append_journal_entry, get_journal, rollback_journal_entry), and activity (acquire_lease, release_lease, heartbeat_lease, get_active_writers) re-exports from db/pg_queries/__init__.py — built-but-unwired infrastructure; underlying files (activity.py, journal.py) kept
• Remove stale "Dependency note" comment in api/routes/bot.py describing status_fn threading as unmerged — that PR (fix: disable MCP in bot subprocess calls; add pipeline invocation logging #32) has been merged

Test plan

• Zero production callers for removed re-exports (verified with grep pre-commit)
• Full test suite passes (pre-existing ical failure unrelated): python -m pytest tests/ --ignore=tests/ical -x -q