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/openapi.json

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