fix: allow tenant slug reuse after soft-delete

Replace absolute UNIQUE constraint on tenants.slug with a partial unique
index that excludes DELETED rows. This allows re-creating a tenant with
the same slug after deletion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/resources/db/migration/V014__tenant_slug_partial_unique.sql

@@ -0,0 +1,5 @@
+-- Replace absolute unique constraint on slug with partial unique index
+-- that excludes DELETED tenants, allowing slug reuse after soft-delete.
+ALTER TABLE tenants DROP CONSTRAINT tenants_slug_key;
+DROP INDEX IF EXISTS idx_tenants_slug;
+CREATE UNIQUE INDEX tenants_slug_active_key ON tenants (slug) WHERE status != 'DELETED';