feat: let ops set a miler's legacyuserid through the console

Backfilling the jupiter->Doormile rider mapping otherwise needs a direct
UPDATE against production, since nothing else writes the column.
UpdateMiler already runs behind admin auth and findMilerForConsole's
tenant scoping, which is the right gate for an ops-only field: it is set
for migrated riders and never by the rider themselves.

Confirmed mapping, matched on phone (jupiter userid -> Doormile userid):
852->37 Sivakumar Subramani, 883->38 Rajan A, 897->39 Varun Edward,
950->40 Jayasabesh Kumar S, 1111->41 Murali P, 1114->42 Tamilazhagan K.

852 sat under a different jupiter partner (Xpress-Mdu-Main) than the
other five, which looked like it might put him out of scope. He is not:
he was migrated to tenant 14 / Nagercoil / hub 18, so a Madurai-side
partner is exactly where jupiter would carry him. jupiter's partnerid is
a rider hub, not a tenant, so it cannot be used to decide tenant
membership.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

controllers/adminController.go

@@ -1843,6 +1843,11 @@ func UpdateMiler(c *fiber.Ctx) error {
 		Displayname        string `json:"displayname"`
 		Defaultvehicletype string `json:"defaultvehicletype"`
 		Hubid              *int   `json:"hubid"`
+		// Legacyuserid links this rider to their old jupiter userid, so telemetry
+		// still arriving over the jupiter NATS chain resolves to them. Writable
+		// here because there is no other way to set it — the column is populated
+		// by ops for migrated riders, never by the rider themselves.
+		Legacyuserid *int `json:"legacyuserid"`
 	}
 
 	req := new(MilerUpdate)
@@ -1859,6 +1864,9 @@ func UpdateMiler(c *fiber.Ctx) error {
 	if req.Hubid != nil {
 		profile.Hubid = req.Hubid
 	}
+	if req.Legacyuserid != nil {
+		profile.Legacyuserid = req.Legacyuserid
+	}
 	profile.Updatedat = time.Now()
 
 	if err := db.DB.Save(profile).Error; err != nil {