Subscriptions — NewPipe-style offline subs + chronological feed

User-curated, no YouTube account. Same posture as Watch Later: you
decide what comes back, no algorithm involved.

Sidecar SubscriptionsFeed op:
  - Takes channel_ids: Vec<String>, per_channel + limit defaults 8/60
  - Caps fan-out at 200 channels per call + each id at 64 chars to
    keep a malicious caller from hammering YouTube via the sidecar
  - Spawns one tokio::task per channel against
    rustypipe::query().channel_videos(), merges results once all
    finish, sorts by publish_date string newest-first
  - A channel that 404s / region-blocks / rustypipe-errors is silently
    dropped — one dead subscription doesn't kill the whole feed; the
    failed list is returned in channels_failed for logging

Addon side:
  - subscriptions.json under addon_data, persisted via same
    _atomic_write_json + _with_lock helpers as Watch Later (no
    repeat of the race + torn-write hazards the audit caught)
  - Two new root menu entries (visible only when subscribed):
    * 'Subscriptions Feed  (N)' — chronological merge of latest uploads
    * 'Subscriptions (channel list)' — per-channel browse
  - Context menu on every video result toggles
    'Subscribe to <channel>' / 'Unsubscribe from <channel>' based on
    current sub state
  - Context menu on each entry in the channel list has its own
    'Unsubscribe from <channel>' for direct removal
  - _unsubscribe_action does Container.Refresh only when
    Container.FolderPath contains 'action=subs' (same guard pattern
    we used for wl_remove)

Live smoke (browse-only, no playback, TV in use):
  - Subscribed to LTT (UCXuqSBlHAE6Xw-yeJA0Tunw) + MKBHD
    (UCBJycsmduvYEL83R_U4JriQ) via RunPlugin
  - subscriptions.json correctly holds both
  - action=subs shows MKBHD + Linus Tech Tips as channel folders
  - action=subs_feed returns 16 merged items: MKBHD's recent uploads
    plus LTT's
  - Root menu now includes 'Subscriptions Feed  (2)' and
    'Subscriptions (channel list)'

Addon v0.0.17.

sidecar/crates/torttube-sidecar/src/resolve.rs

@@ -76,6 +76,93 @@ pub(crate) async fn channel_videos(channel_id: &str, limit: u32) -> Result<Value
     }))
 }
 
+/// Fetch a subscriptions feed: pull the most recent N videos from each
+/// channel in parallel, merge + sort by publish-date newest-first, cap
+/// the returned union at `limit`. A channel that fails (404, region block,
+/// rustypipe error) is silently dropped — one dead subscription shouldn't
+/// kill the whole feed.
+pub(crate) async fn subscriptions_feed(
+    channel_ids: &[String],
+    per_channel: u32,
+    limit: u32,
+) -> Result<Value, HandlerError> {
+    use rustypipe::client::RustyPipe;
+    use std::collections::HashMap;
+
+    let rp = std::sync::Arc::new(RustyPipe::new());
+
+    // Spawn one tokio task per channel; merge results once they all finish.
+    // Fan-out is capped at 200 by main.rs; per_channel capped at 50.
+    let tasks: Vec<_> = channel_ids
+        .iter()
+        .map(|cid| {
+            let rp = rp.clone();
+            let cid = cid.clone();
+            tokio::spawn(async move {
+                let res = rp.query().channel_videos(&cid).await;
+                (cid, res)
+            })
+        })
+        .collect();
+
+    let mut all_items: Vec<Value> = Vec::new();
+    let mut channel_names: HashMap<String, String> = HashMap::new();
+    let mut failed: Vec<String> = Vec::new();
+
+    for handle in tasks {
+        let (cid, res) = match handle.await {
+            Ok(t) => t,
+            Err(e) => {
+                tracing::warn!(error = %e, "subscriptions feed task panicked");
+                continue;
+            }
+        };
+        match res {
+            Ok(ch) => {
+                if !ch.name.is_empty() {
+                    channel_names.insert(cid.clone(), ch.name.clone());
+                }
+                for vi in ch.content.items.iter().take(per_channel as usize) {
+                    if let Ok(v) = serde_json::to_value(vi) {
+                        all_items.push(v);
+                    }
+                }
+            }
+            Err(e) => {
+                tracing::warn!(channel = %cid, error = %e, "subscriptions feed channel failed");
+                failed.push(cid);
+            }
+        }
+    }
+
+    // Sort newest-first by publish_date. rustypipe VideoItem has an optional
+    // `publish_date` as an RFC3339-ish string when known; fall back to
+    // `publish_date_txt` (relative) ordering by string is meaningless, so we
+    // keep channel-order as a stable secondary sort by leaving channel order
+    // as insertion order and using a stable sort.
+    all_items.sort_by(|a, b| {
+        let ad = a.get("publish_date").and_then(Value::as_str).unwrap_or("");
+        let bd = b.get("publish_date").and_then(Value::as_str).unwrap_or("");
+        bd.cmp(ad) // descending
+    });
+    all_items.truncate(limit as usize);
+
+    tracing::info!(
+        channels = channel_ids.len(),
+        items = all_items.len(),
+        failed = failed.len(),
+        "subscriptions_feed ok"
+    );
+
+    Ok(serde_json::json!({
+        "source": "rustypipe",
+        "items": all_items,
+        "channels_total": channel_ids.len(),
+        "channels_failed": failed,
+        "channel_names": channel_names,
+    }))
+}
+
 /// List a playlist's videos. Returns the same VideoItem shape as search/channel.
 pub(crate) async fn playlist(playlist_id: &str, limit: u32) -> Result<Value, HandlerError> {
     use rustypipe::client::RustyPipe;