fix(api): enforce grid occupancy, foreign keys, and atomic derivation writes

[m-murphy]

Summary

• Enable SQLite foreign keys on every database open (reconnects no longer leave FK off).
• Schema version 4: at most one Container per occupied grid position; occupancy is checked on create and move (skipValidation removed).
• Wrap derivation writes in a transaction that stays open across await, including CSV import.
• Integrity report lists containers with no grid position; that list is not counted in the overview issue total.

First API start against an existing file at schema 3 applies migration 004 in place. Startup fails if two occupied cells already share a position.

Test plan

• Schema evolution 3→4 and abort on duplicate occupied cells
• Occupancy rejects tube+tube and tube+well on the same plate
• createDerivation rolls back a derived specimen when child create fails
• Integrity report includes unpositioned containers; overview count excludes them
• Restart API against a backup of the operational SQLite file; SELECT version FROM schema_version is 4