feat(ai): on-device RAG over bookmarks + Spotlight indexing + unified logging

- SpotlightIndexer: actually push BookmarkEntity into the Spotlight index via
  indexAppEntities (IndexedEntity conformance alone indexes nothing). Wired into
  every sync/mutation point. This is what lets Apple Intelligence answer
  free-form Siri questions grounded in the user's bookmarks.
- On-device RAG: BookmarkAssistant (LanguageModelSession) + BookmarkSearchTool
  + SpotlightBookmarkSearch (CSSearchQuery retrieval), surfaced via AskView with
  lightweight Markdown rendering of answers.
- Log: os.Logger facility (com.magicive.marks) replacing ad-hoc print(); shared
  with ShareExtension. ClaudeService now logs server error bodies on non-2xx.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Services/SpotlightIndexer.swift

@@ -0,0 +1,40 @@
+import Foundation
+import CoreSpotlight
+import AppIntents
+
+/// Pushes Linkding bookmarks into the system Spotlight index via the
+/// `IndexedEntity` conformance on `BookmarkEntity`.
+///
+/// This is the step that actually makes bookmarks discoverable: conforming to
+/// `IndexedEntity` only describes *how* an entity would be indexed (its
+/// `attributeSet`) — nothing reaches Spotlight until the entities are handed to
+/// `CSSearchableIndex`. Once indexed, bookmarks show up in Spotlight search and
+/// are reachable by Siri / Apple Intelligence.
+///
+/// All calls are best-effort: failures are logged, never surfaced. If indexing
+/// is unavailable or fails, search degrades but nothing else breaks.
+enum SpotlightIndexer {
+    /// Insert or update the given bookmarks in the Spotlight index.
+    static func index(_ bookmarks: [Bookmark]) async {
+        guard CSSearchableIndex.isIndexingAvailable(), !bookmarks.isEmpty else { return }
+        do {
+            try await CSSearchableIndex.default()
+                .indexAppEntities(bookmarks.map(BookmarkEntity.init(from:)))
+            Log.spotlight.info("Indexed \(bookmarks.count, privacy: .public) bookmarks")
+        } catch {
+            Log.spotlight.error("Index failed: \(error.localizedDescription, privacy: .public)")
+        }
+    }
+
+    /// Remove bookmarks from the index by id (e.g. after delete / archive).
+    static func remove(ids: [Int]) async {
+        guard CSSearchableIndex.isIndexingAvailable(), !ids.isEmpty else { return }
+        do {
+            try await CSSearchableIndex.default()
+                .deleteAppEntities(identifiedBy: ids, ofType: BookmarkEntity.self)
+            Log.spotlight.info("Removed \(ids.count, privacy: .public) bookmarks")
+        } catch {
+            Log.spotlight.error("Delete failed: \(error.localizedDescription, privacy: .public)")
+        }
+    }
+}