Add business-scoped invoices billed to users.

Introduce invoices.user_id, business template/invoice APIs, and tenant-domain public URLs so business dashboards can issue invoices without platform-scope template mismatches.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/058_invoice_user_id.sql

@@ -0,0 +1,46 @@
+-- Invoices bill a User (not a Business). business_id remains tenant/context.
+
+ALTER TABLE invoices
+  ADD COLUMN IF NOT EXISTS user_id BIGINT;
+
+-- Backfill: business owner, else any business_users row for that business
+UPDATE invoices i
+SET user_id = COALESCE(
+  (
+    SELECT bu.user_id
+    FROM business_users bu
+    WHERE bu.business_id = i.business_id
+      AND bu.is_owner = TRUE
+    ORDER BY bu.user_id
+    LIMIT 1
+  ),
+  (
+    SELECT bu.user_id
+    FROM business_users bu
+    WHERE bu.business_id = i.business_id
+    ORDER BY bu.user_id
+    LIMIT 1
+  )
+)
+WHERE i.user_id IS NULL;
+
+-- Drop any invoices that still have no resolvable user (orphan businesses)
+DELETE FROM invoices WHERE user_id IS NULL;
+
+ALTER TABLE invoices
+  ALTER COLUMN user_id SET NOT NULL;
+
+DO $$ BEGIN
+  ALTER TABLE invoices
+    ADD CONSTRAINT invoices_user_id_fkey
+    FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE RESTRICT;
+EXCEPTION
+  WHEN duplicate_object THEN NULL;
+END $$;
+
+CREATE INDEX IF NOT EXISTS idx_invoices_user_created
+  ON invoices (user_id, created_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_invoices_issuer_business_created
+  ON invoices (issuer_business_id, created_at DESC)
+  WHERE issuer_business_id IS NOT NULL;