feat(migrations): support multi-column unique constraints (ZWEI-2850)

[smartive-nicolai]

Adds a new unique constraint kind to the entity model DSL for declaring composite (multi-column) unique indices, which previously had to be hand-written as raw-SQL knex migrations.

Why

The model DSL already supports single-column uniqueness via the field-level unique: true flag, but composite natural keys (join-table FK pairs, (parentId, name) combos, …) had no declarative form — consumers wrote raw CREATE UNIQUE INDEX migrations by hand. This adds first-class support.

Driven by ZWEI-2850 (zwei-wealth-platform wants ~20 composite unique indices declared in-model).

What

New constraint kind:

constraints: [
  {
    kind: 'unique',
    name: 'customer_relation',
    fields: ['customerId', 'relationId'], // DB column names, like `exclude` elements
    where: '"deleted" = false',           // optional partial predicate
  },
]

• Emitted as a CREATE UNIQUE INDEX (not a UNIQUE constraint) so it can be partial via where — essential for soft-deletable entities, where a soft-deleted row should no longer block re-using the natural key. (Single-column unique: true is unchanged and remains the right tool for one column.)
• The migration generator now reflects existing gm-managed unique indexes from pg_index, deliberately excluding any index backing a constraint (primary keys and field-level unique: true, which knex emits as ADD CONSTRAINT … UNIQUE) — so those stay managed by their own code paths and are never touched here.
• add / drop / recreate-on-change is detected on regenerate, mirroring the existing check / exclude / constraint_trigger diffing. Diff compares the ordered column list + normalized where predicate against pg_get_indexdef output.
• Validation mirrors check/exclude: unknown column names and empty fields are rejected, enforced both at Models construction and at migration generation.

Files

• src/models/model-definitions.ts — new unique kind on ConstraintDefinition + UniqueConstraintDefinition export
• src/models/utils.ts — validateUniqueConstraint
• src/models/models.ts — validate at construction
• src/migrations/generate.ts — reflect existing unique indexes, emit/diff on create + alter paths, helper methods
• docs/docs/2-models.md — documents the new kind
• tests/unit/migration-constraints.spec.ts — 7 new unit tests

Verification

• tsc clean
• npm run lint clean
• full unit suite 96/96 pass (incl. the 7 new)

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 27.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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