[CRITICAL] Hardcoded relative database paths crash if CWD is wrong

DATABASE = "db/database.db" and SCHEMA_FILE = "schema.sql" are relative paths that only work if Flask is started from Backend/ directory. Running from any other directory silently creates database in wrong location or raises FileNotFoundError.\n\nFile: Backend/db/db.py, lines 5-6\nFix: Use os.path.join(os.path.dirname(os.path.abspath(__file__)), ...) for absolute paths.