Per-folder "Check all" action + DB backup download

Two small follow-ups to the folder hierarchy that round out the
folder-as-a-profile workflow and give the user a one-click way to keep
an offsite copy of their library state.

Per-folder check
- New `POST /api/folders/:id/check` mirrors the existing
  /api/scheduler/run endpoint but scoped to one folder's members.
  Each channel's stored DownloadOptions (quality / rate limit /
  match-filter / extra args) applies just like a scheduled re-check.
  Returns 409 when downloads are already running.
- Web folder-manager modal: each non-empty folder gets a "⬇ Check"
  button next to Rename / Delete. Status line reports the count of
  channels queued.
- Desktop folder-manager window: same — a small ⬇ icon-button per row,
  visible only for folders with at least one member. Same options-
  honouring re-check path as the right-click "Check for new videos"
  action.
- This effectively gives us "Profiles" lite: organize channels into a
  folder, then trigger a batch re-check whenever you want.

DB backup
- New `GET /api/backup/db` reads `<channels_root>/yt-offline.db` and
  streams it back with `Content-Disposition: attachment` so the
  browser downloads instead of rendering. Filename includes the unix
  timestamp so downloads don't clobber. Returns 404 when running in
  in-memory mode (no DB file to dump).
- We deliberately don't bundle config.toml + cookies.txt here:
  config is short to recreate and cookies.txt carries live session
  credentials that shouldn't fly over the wire unprompted.
- Web settings modal gains a "⬇ Download library snapshot" button in
  a new Backup section, with a hint explaining what's covered (watched
  / favourites / bookmarks / waiting / channel-options / folders).

55 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/web_ui/index.html

@@ -690,6 +690,7 @@ async function openFolderManager(){
     const memberCount=library.filter(c=>c.folder_id===f.id).length;
     return `<div style="display:flex;align-items:center;gap:8px;padding:6px 0;border-bottom:1px solid var(--border)">
       <span style="flex:1">📁 ${esc(f.name)} <span style="color:var(--muted);font-size:11px">(${memberCount} channel${memberCount===1?'':'s'})</span></span>
+      ${memberCount>0?`<button onclick="checkFolder(${f.id},'${esc(f.name)}',this)" title="Check every channel in this folder for new videos">⬇ Check</button>`:''}
       <button onclick="renameFolderPrompt(${f.id},'${esc(f.name)}')">Rename</button>
       <button onclick="deleteFolderConfirm(${f.id},'${esc(f.name)}',${memberCount})" style="color:#f87171">Delete</button>
     </div>`;
@@ -731,6 +732,16 @@ async function renameFolderPrompt(id,oldName){
     openFolderManager();
   }catch(e){setStatus('Error: '+e.message)}
 }
+async function checkFolder(id,name,btn){
+  btn.disabled=true;
+  try{
+    const r=await fetch(`/api/folders/${id}/check`,{method:'POST'});
+    const t=await r.text();
+    if(!r.ok&&r.status!==409)throw new Error(t);
+    setStatus(r.status===409?`${name}: downloads already running`:`${name}: ${t}`);
+  }catch(e){setStatus('Error: '+e.message)}
+  finally{btn.disabled=false}
+}
 async function deleteFolderConfirm(id,name,memberCount){
   const msg=memberCount>0
     ? `Delete folder "${name}"? Its ${memberCount} channel${memberCount===1?'':'s'} will revert to "Unfiled".`
@@ -1072,6 +1083,12 @@ async function openSettings(){
         <span id="plex-status" style="font-size:11px;color:var(--muted)"></span>
       </div>
     </div>
+    <hr style="border-color:var(--border);margin:12px 0">
+    <div style="font-weight:700;margin-bottom:8px">Backup</div>
+    <div class="settings-row" style="flex-direction:column;align-items:flex-start;gap:6px">
+      <div class="settings-hint">Saves the SQLite file holding your watched / favourites / bookmarks / waiting / channel-options / folders. Restore by replacing <code>yt-offline.db</code> in the channels directory.</div>
+      <button onclick="downloadBackupDb()">⬇ Download library snapshot</button>
+    </div>
     ${srcRow}
     <div style="display:flex;gap:8px;justify-content:flex-end;margin-top:12px">
       ${logoutBtn}
@@ -1113,6 +1130,12 @@ async function clearCookies(btn){
   }catch(e){setStatus('Error: '+e.message)}finally{btn.disabled=false}
 }
 
+function downloadBackupDb(){
+  // Open in a new tab so the auth cookie rides along; the server sets
+  // Content-Disposition so the browser triggers a download instead of
+  // rendering the SQLite bytes inline.
+  window.open('/api/backup/db','_blank');
+}
 async function generatePlex(btn){
   const path=document.getElementById('cf-plex-path')?.value.trim();
   if(!path){document.getElementById('plex-status').textContent='Set a path first';return}