feat(tools): add background embedding queue for thoughts

* Implement QueueThought method in BackfillTool for embedding generation
* Update CaptureTool to utilize embedding queuer for failed embeddings
* Add EmbeddingStatus field to Thought type for tracking embedding state

internal/tools/backfill.go

@@ -51,6 +51,30 @@ func NewBackfillTool(db *store.DB, provider ai.Provider, sessions *session.Activ
 	return &BackfillTool{store: db, provider: provider, sessions: sessions, logger: logger}
 }
 
+// QueueThought queues a single thought for background embedding generation.
+// It is used by capture when the embedding provider is temporarily unavailable.
+func (t *BackfillTool) QueueThought(ctx context.Context, id uuid.UUID, content string) {
+	go func() {
+		vec, err := t.provider.Embed(ctx, content)
+		if err != nil {
+			t.logger.Warn("background embedding retry failed",
+				slog.String("thought_id", id.String()),
+				slog.String("error", err.Error()),
+			)
+			return
+		}
+		model := t.provider.EmbeddingModel()
+		if err := t.store.UpsertEmbedding(ctx, id, model, vec); err != nil {
+			t.logger.Warn("background embedding upsert failed",
+				slog.String("thought_id", id.String()),
+				slog.String("error", err.Error()),
+			)
+			return
+		}
+		t.logger.Info("background embedding retry succeeded", slog.String("thought_id", id.String()))
+	}()
+}
+
 func (t *BackfillTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in BackfillInput) (*mcp.CallToolResult, BackfillOutput, error) {
 	limit := in.Limit
 	if limit <= 0 {