feat(118): fakturownia single instance

Phase 118 complete:
- migration 20260512_000109 adds single global Fakturownia settings row
- FakturowniaIntegrationRepository simplified to one-instance API
- FakturowniaIntegrationController + edit view collapsed to one settings page
- Integrations hub shows Fakturownia as single instance
- Invoice config delegated flow always uses global integration_id

Note: shared routes/web.php and DOCS/* updates from Phase 118 are bundled
into the follow-up feat(121+122) commit because Phase 121/122 modified the
same files; hunk-level split was not performed.

Co-Authored-By: Claude <noreply@anthropic.com>

database/migrations/20260512_000109_fakturownia_single_instance.sql

@@ -0,0 +1,74 @@
+INSERT INTO `integrations` (`type`, `name`, `base_url`, `timeout_seconds`, `is_active`, `created_at`, `updated_at`)
+SELECT 'fakturownia', 'Fakturownia', 'https://app.fakturownia.pl', 15, 1, NOW(), NOW()
+WHERE NOT EXISTS (
+    SELECT 1 FROM `integrations` WHERE `type` = 'fakturownia'
+);
+
+SET @fakturownia_integration_id := (
+    SELECT `id`
+    FROM (
+        SELECT
+            i.`id`,
+            i.`is_active`,
+            COALESCE(usage_counts.`used_count`, 0) AS `used_count`
+        FROM `integrations` i
+        LEFT JOIN (
+            SELECT `integration_id`, COUNT(*) AS `used_count`
+            FROM `invoice_configs`
+            WHERE `integration_id` IS NOT NULL
+            GROUP BY `integration_id`
+        ) usage_counts ON usage_counts.`integration_id` = i.`id`
+        WHERE i.`type` = 'fakturownia'
+        ORDER BY i.`is_active` DESC, COALESCE(usage_counts.`used_count`, 0) DESC, i.`id` ASC
+        LIMIT 1
+    ) selected_fakturownia
+);
+
+UPDATE `integrations`
+SET `name` = CONCAT(`name`, ' #', `id`),
+    `updated_at` = NOW()
+WHERE `type` = 'fakturownia'
+  AND `id` <> @fakturownia_integration_id
+  AND `name` = 'Fakturownia';
+
+UPDATE `integrations`
+SET `name` = 'Fakturownia',
+    `base_url` = 'https://app.fakturownia.pl',
+    `timeout_seconds` = 15,
+    `updated_at` = NOW()
+WHERE `id` = @fakturownia_integration_id
+  AND `type` = 'fakturownia';
+
+UPDATE `invoice_configs`
+SET `integration_id` = @fakturownia_integration_id,
+    `updated_at` = NOW()
+WHERE `is_delegated` = 1;
+
+UPDATE `invoice_configs`
+SET `integration_id` = NULL,
+    `updated_at` = NOW()
+WHERE `is_delegated` = 0;
+
+DELETE FROM `fakturownia_integration_settings`
+WHERE `integration_id` <> @fakturownia_integration_id;
+
+INSERT INTO `fakturownia_integration_settings`
+    (`id`, `integration_id`, `account_prefix`, `default_kind`, `default_payment_to_days`, `created_at`, `updated_at`)
+SELECT 1, @fakturownia_integration_id, '', 'vat', 7, NOW(), NOW()
+WHERE NOT EXISTS (
+    SELECT 1
+    FROM `fakturownia_integration_settings`
+    WHERE `integration_id` = @fakturownia_integration_id
+);
+
+UPDATE `fakturownia_integration_settings`
+SET `id` = 1,
+    `integration_id` = @fakturownia_integration_id,
+    `updated_at` = NOW()
+WHERE `integration_id` = @fakturownia_integration_id;
+
+DELETE FROM `integrations`
+WHERE `type` = 'fakturownia'
+  AND `id` <> @fakturownia_integration_id;
+
+ALTER TABLE `fakturownia_integration_settings` AUTO_INCREMENT = 2;