feat(settings,index): per-user index sub-path, strip dead PP settings, scope duplicates

Lets a user pick a sub-folder under their library as a working index root,
stored server-side (new sidecar user_prefs table). The Library tree, reindex,
and both duplicate views (stacks + cross-folder scan) now re-root to it via a
single userLibraryBase() helper. Also fixes the cross-folder scan/archive
endpoints, which previously walked/touched the whole originals root instead
of being scoped per-user (archive now rejects out-of-scope paths, 403).

Removes PhotoPrism settings (Search/Maps/Server-UI/Features/Import) that only
steered PhotoPrism's own bundled SPA and were never read by mulimage's UI.

Also fixes the Library tree occasionally getting stuck on "Loading folders…"
by dropping gcTime:0 and gating the spinner on isLoading instead of isPending.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sidecar/db.go

@@ -26,6 +26,21 @@ type Mark struct {
 // nothing surprising lands in the schema.
 func (Mark) TableName() string { return "marks" }
 
+// UserPref holds the per-user, server-side preferences PhotoPrism's account
+// model has no slot for. Today that's just `IndexPath` — the originals-
+// relative sub-folder (under the user's BasePath) the web client re-roots the
+// Library tree to and scopes the reindex to. Empty string = "whole folder".
+// Keyed by username so each user has independent prefs, matching `Mark`.
+type UserPref struct {
+	UserName  string    `gorm:"primaryKey;size:128;column:user_name" json:"-"`
+	IndexPath string    `gorm:"size:1024;column:index_path" json:"indexPath"`
+	UpdatedAt time.Time `gorm:"column:updated_at" json:"-"`
+}
+
+// TableName pins the table name (GORM would pluralise to `user_prefs` anyway,
+// but pin it explicitly to stay consistent with Mark).
+func (UserPref) TableName() string { return "user_prefs" }
+
 // asJSON returns the wire shape clients expect — same flat object the
 // Node prototype emitted. An empty Mark (rating=nil, color=nil) renders
 // as `{}` which the client treats as "no mark on this photo".
@@ -56,7 +71,7 @@ func openDB(dsn string) (*gorm.DB, error) {
 	if err != nil {
 		return nil, err
 	}
-	if err := db.AutoMigrate(&Mark{}); err != nil {
+	if err := db.AutoMigrate(&Mark{}, &UserPref{}); err != nil {
 		return nil, err
 	}
 	return db, nil