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.

templates/utils.go

@@ -383,6 +383,18 @@ func hxValsFolderDelete(folderPath string) string {
 	return "js:" + string(b)
 }
 
+// hxValsFolderScan builds the hx-vals for the LibraryPanel per-folder
+// Scan button: it scopes the library scan endpoint to ONE registered
+// folder (ScanLibraryRequest.FolderPath). Plain JSON, no js: prefix —
+// the eval-free form under the hash CSP.
+func hxValsFolderScan(folderPath string) string {
+	b, err := json.Marshal(map[string]string{"folder_path": folderPath})
+	if err != nil {
+		return "{}"
+	}
+	return string(b)
+}
+
 // fmtSecondsHMS renders seconds as H:MM:SS (or M:SS under an hour) —
 // the audio progress display shape shared with the web player.
 func fmtSecondsHMS(seconds float64) string {