fix(server): adopt the migrated org on bootstrap; survive fresh install

Two failures found by tracing the migration path against a real upgrade.

Bootstrap orphaned the entire dataset. On upgrade, 0001 creates the
Default org and stamps every legacy document with it, but the instance
has no users, so the operator must run /auth/bootstrap to get in — and
that unconditionally created a SECOND org and put the owner in it. Every
org-scoped read then filtered on the new org, so the operator would log
into an empty Vantage while all their data sat under "default". Nothing
errored and agents kept syncing, so it presented as total data loss.
Bootstrap now adopts the sole existing org, renaming and re-slugging it,
and only creates one when no org exists. More than one org with no users
is refused rather than guessed.

Fresh installs crash-looped. Nothing creates the settings collection
before EnsureSettingsIndexes, so DropOne returned NamespaceNotFound (26),
isIndexNotFound matched only IndexNotFound (27), and that check is fatal.
The same early return also skipped index creation in the secrets and
workflow ensures.

Also: only insert the backfill org on ErrNoDocuments, so a transient read
error can't race the fatal unique slug index; run 0002 before 0003 so the
settings migration can't be pushed into its ambiguous branch; fail 0002's
ambiguous case with a remedy instead of continuing into a fatal index
build; and skip non-string ids in the owner backfill rather than aborting.

server/internal/auth/local.go

@@ -1,9 +1,11 @@
 package auth
 
 import (
+	"fmt"
 	"net/http"
 
 	"github.com/gin-gonic/gin"
+	"github.com/mrhid6/vantage/server/internal/models"
 	"github.com/mrhid6/vantage/server/internal/services"
 )
 
@@ -89,7 +91,36 @@ func HandleBootstrap(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "org_name, email, and password (>=8 chars) required"})
 		return
 	}
-	org, err := services.CreateOrg(body.OrgName)
+	// An upgrade from single-tenant arrives here with no users but with the org
+	// the migrations created and stamped onto every legacy document. Creating a
+	// second org would put the owner somewhere else entirely, and since every
+	// org-scoped read filters on org_id, the operator would land in an empty
+	// Vantage with all their real data still under the migrated org — silent,
+	// total-looking data loss. So adopt the existing org instead, and only
+	// create when there genuinely is none. Same `switch orgCount` shape as
+	// migration 0002.
+	orgCount, err := services.CountOrgs()
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+	var org *models.Org
+	switch orgCount {
+	case 0:
+		org, err = services.CreateOrg(body.OrgName)
+	case 1:
+		var existing *models.Org
+		existing, err = services.FirstOrg()
+		if err == nil {
+			org, err = services.AdoptOrg(existing.OrgID, body.OrgName)
+		}
+	default:
+		c.JSON(http.StatusConflict, gin.H{"error": fmt.Sprintf(
+			"cannot bootstrap: %d organizations already exist but no users do; "+
+				"create the owner against the intended org rather than through setup, "+
+				"or remove the unintended orgs and retry", orgCount)})
+		return
+	}
 	if err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 		return