Handle partial import

[SilasBerger]

Implement a strategy for handling partially imported / stored entries in case of script interruption (crash, killed by user), taking into account the fact that changes may already have been made to other tables as well (hashtags, hashtag rel, ...)

Strategy:

• Single transaction for video metadata, mark entry as pass or fail
• Subsequent second transaction for transcript, if first transaction succeeded
• Ensure on delete cascade constraints work as intended
• Provide DAO functionality for deleting orphan hashtags, challenges, etc.

Reference: #13

[SilasBerger]

Implemented with move to SQLite