FileBrowser: diagnose container mounts + support extra audio path

This is a Docker volume issue, not a code bug. The backend container
only sees paths explicitly mounted in docker-compose.yml. A new mount
on the Linux host (e.g. NAS share, USB drive) is invisible to the
container until added as a volume — restarting the container alone
doesn't help, and restarting just the host doesn't either.

Backend: New GET /api/filebrowser/diagnose endpoint reads
/proc/self/mountinfo and returns the actual bind/nfs/cifs mounts the
container sees, plus a check of common candidate roots (/audiofiles,
/mnt, /media, /srv, /home, /app/data) showing whether they exist and
have content.

Frontend: Info icon in FileBrowser header toggles a diagnose panel
showing mounts and root candidates. Quick-access buttons now built
dynamically from candidate roots that actually exist. On 'path not
found' error: helpful inline explanation including the exact .env
variable and docker-compose command needed to add a new mount.

docker-compose.yml: New EXTRA_AUDIO_PATH env variable. Mounts a
second host path 1:1 (same path inside container, like
AUDIOFILES_PATH does). Defaults to ./data → /extra_audio_unused
when unset, which is a no-op.

.env.example: Documents EXTRA_AUDIO_PATH usage with example.

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

backend/app/routers/filebrowser.py

@@ -35,3 +35,64 @@ async def browse(
     parent = parent_path if parent_path != path else None
 
     return {"path": path, "parent": parent, "entries": entries}
+
+
+@router.get("/diagnose")
+async def diagnose(_admin: User = Depends(require_admin)):
+    """Zeigt was der Backend-Container sieht: Bind-Mounts und Top-Level-Verzeichnisse."""
+    # Aktive Mounts aus /proc/self/mountinfo
+    mounts = []
+    try:
+        with open("/proc/self/mountinfo", "r") as f:
+            for line in f:
+                parts = line.split()
+                # Format: id parent major:minor root mount_point options - fstype source super_options
+                if len(parts) < 10 or "-" not in parts:
+                    continue
+                sep = parts.index("-")
+                mount_point = parts[4]
+                fstype = parts[sep + 1] if sep + 1 < len(parts) else "?"
+                source = parts[sep + 2] if sep + 2 < len(parts) else "?"
+                # Interne Kernel-Mounts überspringen
+                if any(mount_point.startswith(p) for p in ("/proc", "/sys", "/dev", "/etc/", "/run", "/tmp")):
+                    continue
+                if mount_point in ("/", "/etc/resolv.conf", "/etc/hostname", "/etc/hosts"):
+                    continue
+                mounts.append({
+                    "mountPoint": mount_point,
+                    "fsType": fstype,
+                    "source": source,
+                })
+    except Exception as e:
+        mounts_error = str(e)
+    else:
+        mounts_error = None
+
+    # Top-Level-Verzeichnisse die im Container existieren und für Hörbücher in Frage kommen
+    candidates = ["/audiofiles", "/mnt", "/media", "/srv", "/home", "/app/data"]
+    roots = []
+    for p in candidates:
+        if os.path.isdir(p):
+            try:
+                entries = os.listdir(p)
+                roots.append({
+                    "path": p,
+                    "exists": True,
+                    "entryCount": len([e for e in entries if not e.startswith(".")]),
+                })
+            except PermissionError:
+                roots.append({"path": p, "exists": True, "entryCount": None, "permError": True})
+        else:
+            roots.append({"path": p, "exists": False})
+
+    return {
+        "mounts": mounts,
+        "mountsError": mounts_error,
+        "candidateRoots": roots,
+        "hint": (
+            "Pfade die hier NICHT in 'mounts' auftauchen, sind im Container nicht sichtbar. "
+            "Sie müssen in docker-compose.yml als Volume gemountet werden — "
+            "z.B. via EXTRA_AUDIO_PATH=/dein/host/pfad in der .env und dann "
+            "'docker-compose down && docker-compose up -d'."
+        ),
+    }