feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/maintenance.dbml

@@ -0,0 +1,30 @@
+Table maintenance_tasks {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  category text
+  frequency_days int
+  last_completed timestamptz
+  next_due timestamptz
+  priority text [not null, default: 'medium']
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    next_due
+  }
+}
+
+Table maintenance_logs {
+  id uuid [pk, default: `gen_random_uuid()`]
+  task_id uuid [not null, ref: > maintenance_tasks.id]
+  completed_at timestamptz [not null, default: `now()`]
+  performed_by text
+  cost "decimal(10,2)"
+  notes text
+  next_action text
+
+  indexes {
+    (task_id, completed_at)
+  }
+}