fix: add ON DELETE cascade/set-null rules to FK relationships

[jakebromberg]

Summary

• Most FK relationships in wxyc_schema tables defaulted to ON DELETE NO ACTION, blocking legitimate operations like deleting users or schedule entries when child rows exist.
• Added onDelete: 'set null' to 8 FK columns where the child row should be preserved (schedule DJ assignments, flowsheet entries, show primary DJ, shift cover DJ).
• Added onDelete: 'cascade' to 7 FK columns where the child row is meaningless without the parent (rotation/review entries, cross-reference tables, show_djs.show_id).
• Left 4 FK columns as NO ACTION intentionally (library → artists/genres/format, artists → genres) to block deletion when albums/artists still reference them.
• Added unit test (tests/unit/database/schema.fk-cascades.test.ts) that asserts correct onDelete rules for all 19 FK relationships.

Test plan

• Unit test verifies all 15 FK columns have the correct onDelete rule
• Unit test verifies 4 FK columns intentionally lack onDelete (NO ACTION)
• Generate and review migration before applying to production DB

Closes #38

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