Integrate xklb db in Calibre-Web

[deldesir]

This PR is a work in progress and will be broken into several small ones once done. It contains adjustments made to use xklb.db alongside metadata.db and app.db. It minimizes database reopening costs, uses SQLAlchemy so the code structure can be preserved.

[holta]

See discussion evolving at:

• PR #244 [searching] is prone to false positives [ALSO: database integrity questions about safely/synchronously deleting "videos" from both/all DB's] #246

[holta]

This PR is a work in progress and will be broken into several small ones once done. It contains adjustments made to use xklb.db alongside metadata.db and app.db. It minimizes database reopening costs [and uses SQLAlchemy to better align with upstream Calibre-Web, and likewise Calibre upstream of that!]

• Support path of xklb db as argument #248
• PR Set arg x for xklb db path #249
• PR Init xklb db based on arg #250
• PR Set constant for default xklb db file #251
• PR "Attach" xklb db alongside metadata.db and app.db #252
• PR New xb.py: Possible schema to standardize xklb db's + main operations? [need to align w/ Calibre-Web's metadata.db & app.db!] #253
• PR Refactor video_metadata to use xb #254
• PR Begin to establish 1:1 mapping between media_id (xklb-metadata.db) <-> Calibre-Web's book_id (metadata.db) #255