Make CloudKit apply path scope-aware and re-upload local-newer merges

CloudKit record names carry a source scope suffix so the same channel
or video ID can exist under different sources, but the apply path
matched local entities by bare ID: incoming records could merge into an
entity from a different scope, and a remote deletion of one scope
deleted every local entity with that ID and cascaded scoped deletions
for all other scopes back to CloudKit.

- Match local entities on the full mapper-derived record name when
  merging incoming records, materializing records for upload, and
  applying remote deletions.
- Remote deletions now remove only the matching entity and no longer
  queue CloudKit deletions (no echo, no cross-scope cascade).
- Insert dedupe for CloudKit-applied records is scope-aware, so a
  same-ID record from a different source inserts instead of being
  silently dropped. Remotely added bookmarks also update the
  fast-lookup cache immediately.
- When conflict resolution during a fetch keeps newer local data, the
  merge result is queued for upload; previously the server (and other
  devices) kept the stale version unless the entity was edited again.
  Strict timestamp comparisons prevent re-upload ping-pong between
  devices.

Yattee/Data/DataManager+Subscriptions.swift

@@ -200,6 +200,16 @@ extension DataManager {
         }
     }
 
+    /// Gets all subscriptions matching a channel ID. The same ID can exist
+    /// under multiple source scopes; callers that care about a specific
+    /// source must pick the matching entity themselves.
+    func subscriptions(forChannelID channelID: String) -> [Subscription] {
+        let descriptor = FetchDescriptor<Subscription>(
+            predicate: #Predicate { $0.channelID == channelID }
+        )
+        return (try? modelContext.fetch(descriptor)) ?? []
+    }
+
     /// Gets all subscriptions.
     func subscriptions() -> [Subscription] {
         let descriptor = FetchDescriptor<Subscription>(
@@ -219,15 +229,17 @@ extension DataManager {
     /// Inserts a subscription into the database.
     /// Used by SubscriptionService for caching server subscriptions locally.
     func insertSubscription(_ subscription: Subscription) {
-        // Check for duplicates
+        // Check for duplicates within the same source scope - the same
+        // channel ID can legitimately exist under different sources
         let channelID = subscription.channelID
         let descriptor = FetchDescriptor<Subscription>(
             predicate: #Predicate { $0.channelID == channelID }
         )
+        let scopeSuffix = subscription.sourceScopeSuffix
 
         do {
             let existing = try modelContext.fetch(descriptor)
-            if existing.isEmpty {
+            if !existing.contains(where: { $0.sourceScopeSuffix == scopeSuffix }) {
                 modelContext.insert(subscription)
                 save()
             }
@@ -394,3 +406,17 @@ extension DataManager {
         }
     }
 }
+
+// MARK: - Source Scope
+
+private extension Subscription {
+    /// Record-name scope suffix used to distinguish same-ID entities across sources.
+    var sourceScopeSuffix: String {
+        SourceScope.from(
+            sourceRawValue: sourceRawValue,
+            globalProvider: providerName,
+            instanceURLString: instanceURLString,
+            externalExtractor: nil
+        ).recordNameSuffix
+    }
+}