smarter ids

In the current schema, IDs are implemented as INTs. I think they should be strings. The IDs should then be hashes representing the values of the fields. This has two advantages:

1) no integer overflow (although this might not be a problem if the int has enough bits)
2) automatic duplicate detection: if the hashes match, the content of the entry matches and addition to the database is prevented

No schema file, test, or entry point is named. Start by reviewing the current schema definitions and ID constraints, then clarify which fields form each hash and how duplicate inserts should be handled. The work is done when the ID representation and duplicate-detection behavior are agreed and implemented.

Written by the indexing model from the issue text.