sub: file-level sync (md2notes)

[FeatheredHwang]

create test material, including:

• add
• update/modify
• delete
• no-change

change the material with python codes, to simulate user edit, including:

• add
• update/modify
• delete
• no-change

while join, judge if changed, then do the coresponding job, including:

• if add, join it
• update/modify, modify deck
• delete, bury the related notes
• no-change, do nothing

to achive that, we need to design the code framework/skeleton first:

• Skeleton: create unique file-id #7
• Skeleton: create db
• Skeleton: save ID to file #6
• Skeleton: save ID to db