feat(libraries): scope a library scan to one registered folder

POST /api/libraries/:id/scan accepts an optional {"folder_path": "..."}:
the scan then covers exactly that one registered folder's subtree instead
of the whole library. The path is validated against the library's
registered folders — anything else is a 400, never trusted. An omitted
body keeps today's full-library behavior, and coalescing (409 with the
active job) is unchanged.

The LibraryPanel folder rows get a per-folder Scan button: pure htmx
attributes posting to the same scan endpoint, with hxValsFolderScan
emitting plain eval-free JSON for hx-vals alongside the existing
hxValsFolderDelete precedent. Progress surfaces through the existing
active-scan polling; no new client code.

Safety: the scanner's archive-reconciliation guard (previous commit)
skips partially-covered libraries, so a one-folder scan cannot mark the
library's other folders' items missing.

internal/handlers/scanner.go

@@ -27,7 +27,12 @@ func (h *Handler) StartBackgroundTasks() {
 type ScanLibraryRequest struct {
 	FolderPaths []string `json:"folder_paths,omitempty"`
 	LibraryID   string   `json:"library_id,omitempty"`
-	Force       bool     `json:"force,omitempty"`
+	// FolderPath (singular) scopes a library scan to ONE of the library's
+	// registered folders (the LibraryPanel per-folder Scan buttons). It
+	// must be a registered folder — validated below, never trusted. The
+	// raw /api/scanner/scan route ignores it (no library attribution).
+	FolderPath string `json:"folder_path,omitempty"`
+	Force      bool   `json:"force,omitempty"`
 }
 
 // ScanLibrary handles POST /api/scanner/scan (now runs in background)
@@ -68,9 +73,28 @@ func (h *Handler) ScanLibrary(c *echo.Context) error {
 			return c.JSON(http.StatusNotFound, map[string]string{"error": "library not found or has no folders"})
 		}
 
-		// Extract folder paths
-		for _, folder := range libraryFolders {
-			folderPaths = append(folderPaths, folder.FolderPath)
+		if req.FolderPath != "" {
+			// Per-folder scope (LibraryPanel per-folder Scan buttons):
+			// scan exactly one registered folder's subtree. The archive
+			// reconciliation pass skips partially-covered libraries
+			// (processScanJob → ScanFolders), so the other folders'
+			// items cannot be marked missing by this run.
+			registered := false
+			for _, folder := range libraryFolders {
+				if folder.FolderPath == req.FolderPath {
+					registered = true
+					break
+				}
+			}
+			if !registered {
+				return c.JSON(http.StatusBadRequest, map[string]string{"error": "folder_path is not one of this library's registered folders"})
+			}
+			folderPaths = []string{req.FolderPath}
+		} else {
+			// Extract folder paths
+			for _, folder := range libraryFolders {
+				folderPaths = append(folderPaths, folder.FolderPath)
+			}
 		}
 
 		if len(folderPaths) == 0 {