Fix thread safety and resource leaks in DB class

[Copilot]

Addresses review feedback on src/db.py: class-level lock causes unnecessary serialization across DB instances, and connections leak when login=True.

Changes

• Instance-level locking: Moved _lock from class to instance attribute. Each DB instance now has its own lock, eliminating cross-instance contention when operating on separate database files.

• Resource leak fix: Connections created with login=True are now properly closed in finally blocks. Previously, temporary connections were never closed.

• APSW compatibility: Removed cursor context managers (with cursor:). APSW cursors don't support the context manager protocol and are lightweight - they don't require explicit cleanup.

def __insert_or_delete(self, query, params, login=False):
    conn = None
    try:
        if login:
            conn = DB.__create_connection(self.__dbfile)
            cursor = conn.cursor()
        else:
            cursor = self.__conn.cursor()
        self.__log_and_execute(cursor, query, params)
        return 1
    except apsw.Error as err:
        log.exception("Database Error: %s" % err)
        return 0
    finally:
        if conn is not None:
            conn.close()  # Prevents resource leak

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