Custom metadata: update Statistics db on metadata change

[poire-z]

Followup to #10861 and #10869 : update metadata in Reading Statistics DB when they change, so that the book can be found on the next opening and previous statistics are not lost - and so that the updated title/authors are reflected in the reading statistics (calendar view, today timeline & the others).

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This change is 

[hius07]

All looks very good.
metadata_value_new is not used, I will remove it.
I'm thinking of imposing some restrictions on series_index, it should not exist without series itself.

[poire-z]

I'm thinking of imposing some restrictions on series_index, it should not exist without series itself.

As you feel (I hardly ever have books with series, and I don't really know/care about how this works/looks).

[Frenzie]

@poire-z On a completely unrelated note, your beautiful calendar is problematic. It's made me lazy about properly logging what I'm reading. ;-)

[poire-z]

:)
I still log my reading (and movies watching) to a text file - easier to grep so I don't buy or reread the same books :)
So, as long as Statistics does not have a find feature, logging is still needed :)

[Frenzie]

I definitely still do that, just a bit late sometimes.