revert(maintenance): keep one-time TTL cleanup, narrow PR to validator-only

Per review feedback: auto-deleting completed one-time maintenance windows is
intentional cleanup, not a bug. The actual bug is past-dated end values being
accepted at creation time, which the validator refines now reject up front.

Reverts:
- Restore expiry field + TTL index on MaintenanceWindow schema
- Restore repository's auto-set of expiry = end for repeat=0
- Drop migration 0007 (no longer needed)
- Restore expiry properties in openapi.json
- Restore expiry in create/edit validators (still optional)

Kept from this PR:
- .strict() on create/edit body schemas
- end > start refine on create/edit
- end > now refine on create when repeat=0 (the real fix)
- DurationUnits const tuple + reuse (rules 5/8/10)
- Client no longer sends redundant expiry (server still derives it)

server/src/validation/maintenanceWindowValidation.ts

@@ -18,6 +18,7 @@ export const createMaintenanceWindowBodyValidation = z
 		start: dateToString,
 		end: dateToString,
 		repeat: z.number().min(0, "Repeat must be a non-negative number"),
+		expiry: dateToString.optional(),
 	})
 	.strict()
 	.superRefine((data, ctx) => {
@@ -70,6 +71,7 @@ export const editMaintenanceByIdWindowBodyValidation = z
 		repeat: z.number().optional(),
 		start: dateToString.optional(),
 		end: dateToString.optional(),
+		expiry: dateToString.optional(),
 		monitors: z.array(z.string()).optional(),
 		duration: z.number().optional(),
 		durationUnit: z.enum(DurationUnits).optional(),