refactor(maintenance): centralize DurationUnit + sync OpenAPI spec

Convention pass per docs/coding-conventions.md (rules 5/8/10/11):
- Promote DurationUnit from a raw union type to a const tuple + derived type.
- Mongoose schema enum imports DurationUnits instead of inlining the values.
- Both validators import DurationUnits instead of inlining z.enum twice.
- Drop stale 'expiry' entries from openapi.json (spec is now strict-mode-correct).

server/openapi.json

@@ -4657,11 +4657,6 @@
 									"repeat": {
 										"type": "number",
 										"minimum": 0
-									},
-									"expiry": {
-										"type": "string",
-										"nullable": true,
-										"format": "date-time"
 									}
 								},
 								"required": ["monitors", "name", "duration", "durationUnit", "start", "end", "repeat"]
@@ -5190,11 +5185,6 @@
 										"nullable": true,
 										"format": "date-time"
 									},
-									"expiry": {
-										"type": "string",
-										"nullable": true,
-										"format": "date-time"
-									},
 									"monitors": {
 										"type": "array",
 										"items": {