feat: comprehensive health check with DB, scheduler, cache

- Add /api/v1/health endpoint with component-level checks
- Verify DB connectivity, fail2ban socket, scheduler, session cache
- Add SQLite WAL cleanup on startup (orphan crash files)
- Migration 8: import_log.timestamp → INTEGER UNIX epoch
- Align import_log timestamps with history_archive (already UNIX int)
- Add unit tests for DB cleanup and health router

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_db.py

@@ -7,6 +7,7 @@ import pytest
 
 from app.db import (
     _apply_migration,
+    _cleanup_wal_files,
     _parse_migration_statements,
     init_db,
     open_db,
@@ -241,3 +242,32 @@ async def test_init_db_idempotent(tmp_path: Path) -> None:
     finally:
         await db.close()
 
+
+async def test_cleanup_wal_files_removes_orphaned_files(tmp_path: Path) -> None:
+    """Test that _cleanup_wal_files removes orphaned WAL and SHM files."""
+    db_path = str(tmp_path / "test_wal.db")
+    wal_path = Path(db_path + "-wal")
+    shm_path = Path(db_path + "-shm")
+
+    # Create the orphaned files
+    wal_path.write_text("orphan")
+    shm_path.write_text("orphan")
+
+    assert wal_path.exists()
+    assert shm_path.exists()
+
+    # Run cleanup
+    await _cleanup_wal_files(db_path)
+
+    # Both files should be removed
+    assert not wal_path.exists()
+    assert not shm_path.exists()
+
+
+async def test_cleanup_wal_files_handles_missing_files(tmp_path: Path) -> None:
+    """Test that _cleanup_wal_files handles non-existent files gracefully."""
+    db_path = str(tmp_path / "nonexistent.db")
+
+    # Should not raise
+    await _cleanup_wal_files(db_path)
+