Fix inconsistent FK ON DELETE behavior on bulk_operations/ticket_templates (#40)

bulk_operations.performed_by and ticket_templates.created_by had no
ON DELETE clause (defaulting to RESTRICT), unlike every other
user-reference FK in the schema (tickets.*, ticket_attachments,
ticket_dependencies, recurring_tickets, api_keys), which all use
SET NULL. Deleting a user who ever ran a bulk operation or created a
template hard-failed at the DB level instead of nulling the
reference, breaking the pattern used everywhere else.

performed_by was NOT NULL, so it had to become nullable to support
SET NULL, matching how every other SET NULL column is defined.

- Fixed 000_baseline.sql for fresh installs.
- Added 003_fk_on_delete_set_null.sql for existing deployments.

Verified against a local MariaDB instance: reproduced the old RESTRICT
schema, ran the migration (twice, for idempotency), then confirmed
deleting a user with rows in both tables now nulls the references
instead of failing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

migrations/000_baseline.sql

@@ -57,7 +57,7 @@ CREATE TABLE IF NOT EXISTS `bulk_operations` (
   `operation_id` int(11) NOT NULL AUTO_INCREMENT,
   `operation_type` varchar(50) NOT NULL,
   `ticket_ids` text NOT NULL,
-  `performed_by` int(11) NOT NULL,
+  `performed_by` int(11) DEFAULT NULL,
   `parameters` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL CHECK (json_valid(`parameters`)),
   -- 32, not 20: 'completed_with_errors' is 21 chars (see 001_widen_bulk_operations_status.sql)
   `status` varchar(32) DEFAULT 'pending',
@@ -69,7 +69,7 @@ CREATE TABLE IF NOT EXISTS `bulk_operations` (
   PRIMARY KEY (`operation_id`),
   KEY `idx_performed_by` (`performed_by`),
   KEY `idx_created_at` (`created_at`),
-  CONSTRAINT `bulk_operations_ibfk_1` FOREIGN KEY (`performed_by`) REFERENCES `users` (`user_id`)
+  CONSTRAINT `bulk_operations_ibfk_1` FOREIGN KEY (`performed_by`) REFERENCES `users` (`user_id`) ON DELETE SET NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
 
 -- ============ custom_field_definitions ============
@@ -238,7 +238,7 @@ CREATE TABLE IF NOT EXISTS `ticket_templates` (
   PRIMARY KEY (`template_id`),
   KEY `created_by` (`created_by`),
   KEY `idx_template_name` (`template_name`),
-  CONSTRAINT `ticket_templates_ibfk_1` FOREIGN KEY (`created_by`) REFERENCES `users` (`user_id`)
+  CONSTRAINT `ticket_templates_ibfk_1` FOREIGN KEY (`created_by`) REFERENCES `users` (`user_id`) ON DELETE SET NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
 
 -- ============ ticket_watchers ============