feat: add document invite handler

cmd/server/main.go

···
 	mux.HandleFunc("POST /api/docs/{rkey}/save", h.APIDocumentSave)
 	mux.HandleFunc("PUT /api/docs/{rkey}/autosave", h.APIDocumentAutoSave)
 	mux.HandleFunc("DELETE /api/docs/{rkey}", h.APIDocumentDelete)
+	mux.HandleFunc("POST /api/docs/{rkey}/invite", h.DocumentInvite)
 
 	// Middleware stack
 	stack := middleware.Logger(

internal/handler/handler.go

···
 	"html/template"
 	"log"
 	"net/http"
+	"os"
 	"regexp"
 	"strings"
 	"time"
 
 	"github.com/limeleaf/diffdown/internal/atproto/xrpc"
 	"github.com/limeleaf/diffdown/internal/auth"
+	"github.com/limeleaf/diffdown/internal/collaboration"
 	"github.com/limeleaf/diffdown/internal/db"
 	"github.com/limeleaf/diffdown/internal/model"
 	"github.com/limeleaf/diffdown/internal/render"
···
 	}
 
 	h.jsonResponse(w, map[string]string{"status": "ok"})
+}
+
+// DocumentInvite creates an invite link for a document.
+func (h *Handler) DocumentInvite(w http.ResponseWriter, r *http.Request) {
+	user := h.currentUser(r)
+	if user == nil {
+		http.Redirect(w, r, "/auth/login", http.StatusSeeOther)
+		return
+	}
+
+	rkey := r.PathValue("rkey")
+	if rkey == "" {
+		http.Error(w, "Invalid document", http.StatusBadRequest)
+		return
+	}
+
+	client, err := h.xrpcClient(user.ID)
+	if err != nil {
+		log.Printf("DocumentInvite: xrpc client: %v", err)
+		h.render(w, "document_edit.html", PageData{Error: "Not authenticated with ATProto"})
+		return
+	}
+
+	value, _, err := client.GetRecord(client.DID(), collectionDocument, rkey)
+	if err != nil {
+		http.Error(w, "Document not found", http.StatusNotFound)
+		return
+	}
+
+	doc := &model.Document{}
+	if err := json.Unmarshal(value, doc); err != nil {
+		http.Error(w, "Invalid document", http.StatusInternalServerError)
+		return
+	}
+	doc.RKey = rkey
+
+	session, err := h.DB.GetATProtoSession(user.ID)
+	if err != nil || session == nil {
+		http.Error(w, "Unauthorized", http.StatusForbidden)
+		return
+	}
+
+	ownerDID := strings.TrimPrefix(doc.URI, "at://")
+	if idx := strings.Index(ownerDID, "/"); idx > 0 {
+		ownerDID = ownerDID[:idx]
+	}
+	if ownerDID == "" || session.DID != ownerDID {
+		http.Error(w, "Unauthorized", http.StatusForbidden)
+		return
+	}
+
+	if len(doc.Collaborators) >= 5 {
+		http.Error(w, "Maximum collaborators reached", http.StatusBadRequest)
+		return
+	}
+
+	invite, err := collaboration.CreateInvite(h.DB, rkey, session.DID)
+	if err != nil {
+		log.Printf("DocumentInvite: create invite: %v", err)
+		http.Error(w, "Failed to create invite", http.StatusInternalServerError)
+		return
+	}
+
+	baseURL := os.Getenv("BASE_URL")
+	if baseURL == "" {
+		baseURL = "http://127.0.0.1:8080"
+	}
+	inviteLink := fmt.Sprintf("%s/doc/%s?invite=%s", baseURL, rkey, invite.Token)
+	h.render(w, "document_edit.html", PageData{
+		Title: "Edit " + doc.Title,
+		User:  user,
+		Content: map[string]interface{}{
+			"document":   doc,
+			"inviteLink": inviteLink,
+		},
+	})
 }
 
 // --- API: Render markdown ---