feat: incremental SSE scan, recursive NC traversal, custom folder support

- nc_scan.py: recursive collect_audio_files (fixes depth-1 bug); scan_band_folder
  yields ndjson events (progress/song/session/skipped/done) for streaming
- songs.py: replace old flat scan with scan_band_folder; add GET nc-scan/stream
  endpoint using _member_from_request so ?token= auth works for fetch-based SSE
- BandPage.tsx: scan button now consumes ndjson stream via fetch+ReadableStream;
  sessions/unattributed invalidated as each song/session event arrives
- session.py: add extract_session_folder() for YYMMDD path extraction
- rehearsal_session.py: get_or_create uses begin_nested() savepoint to handle races
- band.py: add get_by_nc_folder_prefix() for custom nc_folder_path band lookup
- internal.py: nc-upload falls back to prefix match when slug lookup fails
- event_loop.py: remove hardcoded bands/ guard; let internal API handle filtering

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/pages/BandPage.tsx

@@ -60,6 +60,8 @@ export function BandPage() {
   const [showCreate, setShowCreate] = useState(false);
   const [title, setTitle] = useState("");
   const [error, setError] = useState<string | null>(null);
+  const [scanning, setScanning] = useState(false);
+  const [scanProgress, setScanProgress] = useState<string | null>(null);
   const [scanMsg, setScanMsg] = useState<string | null>(null);
   const [inviteLink, setInviteLink] = useState<string | null>(null);
   const [editingFolder, setEditingFolder] = useState(false);
@@ -123,22 +125,65 @@ export function BandPage() {
     onError: (err) => setError(err instanceof Error ? err.message : "Failed to create song"),
   });
 
-  const scanMutation = useMutation({
-    mutationFn: () => api.post<NcScanResult>(`/bands/${bandId}/nc-scan`, {}),
-    onSuccess: (result) => {
-      qc.invalidateQueries({ queryKey: ["sessions", bandId] });
-      qc.invalidateQueries({ queryKey: ["songs-unattributed", bandId] });
-      if (result.imported > 0) {
-        setScanMsg(`Imported ${result.imported} new song${result.imported !== 1 ? "s" : ""} from ${result.folder} (${result.skipped} already registered).`);
-      } else if (result.files_found === 0) {
-        setScanMsg(`No audio files found in ${result.folder}.`);
-      } else {
-        setScanMsg(`All ${result.files_found} file${result.files_found !== 1 ? "s" : ""} in ${result.folder} already registered.`);
+  async function startScan() {
+    if (scanning || !bandId) return;
+    setScanning(true);
+    setScanMsg(null);
+    setScanProgress("Starting scan…");
+
+    const token = localStorage.getItem("rh_token");
+    const url = `/api/v1/bands/${bandId}/nc-scan/stream${token ? `?token=${encodeURIComponent(token)}` : ""}`;
+
+    try {
+      const resp = await fetch(url);
+      if (!resp.ok || !resp.body) {
+        const text = await resp.text().catch(() => resp.statusText);
+        throw new Error(text || `HTTP ${resp.status}`);
       }
-      setTimeout(() => setScanMsg(null), 6000);
-    },
-    onError: (err) => setScanMsg(err instanceof Error ? err.message : "Scan failed"),
-  });
+
+      const reader = resp.body.getReader();
+      const decoder = new TextDecoder();
+      let buf = "";
+
+      while (true) {
+        const { done, value } = await reader.read();
+        if (done) break;
+        buf += decoder.decode(value, { stream: true });
+        const lines = buf.split("\n");
+        buf = lines.pop() ?? "";
+
+        for (const line of lines) {
+          if (!line.trim()) continue;
+          let event: Record<string, unknown>;
+          try { event = JSON.parse(line); } catch { continue; }
+
+          if (event.type === "progress") {
+            setScanProgress(event.message as string);
+          } else if (event.type === "song" || event.type === "session") {
+            qc.invalidateQueries({ queryKey: ["sessions", bandId] });
+            qc.invalidateQueries({ queryKey: ["songs-unattributed", bandId] });
+          } else if (event.type === "done") {
+            const s = event.stats as { found: number; imported: number; skipped: number };
+            if (s.imported > 0) {
+              setScanMsg(`Imported ${s.imported} new song${s.imported !== 1 ? "s" : ""} (${s.skipped} already registered).`);
+            } else if (s.found === 0) {
+              setScanMsg("No audio files found.");
+            } else {
+              setScanMsg(`All ${s.found} file${s.found !== 1 ? "s" : ""} already registered.`);
+            }
+            setTimeout(() => setScanMsg(null), 6000);
+          } else if (event.type === "error") {
+            setScanMsg(`Scan error: ${event.message}`);
+          }
+        }
+      }
+    } catch (err) {
+      setScanMsg(err instanceof Error ? err.message : "Scan failed");
+    } finally {
+      setScanning(false);
+      setScanProgress(null);
+    }
+  }
 
   const inviteMutation = useMutation({
     mutationFn: () => api.post<BandInvite>(`/bands/${bandId}/invites`, {}),
@@ -306,11 +351,11 @@ export function BandPage() {
           <h2 style={{ color: "var(--text)", margin: 0, fontSize: 16 }}>Recordings</h2>
           <div style={{ display: "flex", gap: 8 }}>
             <button
-              onClick={() => scanMutation.mutate()}
-              disabled={scanMutation.isPending}
+              onClick={startScan}
+              disabled={scanning}
               style={{ background: "none", border: "1px solid var(--border)", borderRadius: 6, color: "var(--teal)", cursor: "pointer", padding: "6px 14px", fontSize: 12 }}
             >
-              {scanMutation.isPending ? "Scanning…" : "⟳ Scan Nextcloud"}
+              {scanning ? "Scanning…" : "⟳ Scan Nextcloud"}
             </button>
             <button
               onClick={() => { setShowCreate(!showCreate); setError(null); }}
@@ -321,6 +366,11 @@ export function BandPage() {
           </div>
         </div>
 
+        {scanning && scanProgress && (
+          <div style={{ background: "var(--bg-subtle)", border: "1px solid var(--border)", borderRadius: 6, color: "var(--text-muted)", fontSize: 12, padding: "8px 14px", marginBottom: 8, fontFamily: "monospace" }}>
+            {scanProgress}
+          </div>
+        )}
         {scanMsg && (
           <div style={{ background: "var(--teal-bg)", border: "1px solid var(--teal)", borderRadius: 6, color: "var(--teal)", fontSize: 12, padding: "8px 14px", marginBottom: 12 }}>
             {scanMsg}