feat: implement Story 3.1 Open Registration

Add complete implementation with tests:
- New route POST /admin/exchange/<id>/state/open-registration
- State validation (only from draft state)
- Success/error messages
- Authentication required
- Update exchange detail template with "Open Registration" button
- 8 comprehensive integration tests

All acceptance criteria met:
- "Open Registration" action available from Draft state
- Exchange state changes to "Registration Open"
- Registration link becomes active
- Participants can now access registration form
- Success message displayed
- Only admin can perform action
- Redirects to exchange detail after completion

Story: 3.1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/admin.py

@@ -180,3 +180,34 @@ def view_exchange(exchange_id):
     """
     exchange = db.session.query(Exchange).get_or_404(exchange_id)
     return render_template("admin/exchange_detail.html", exchange=exchange)
+
+
+@admin_bp.route("/exchange/<int:exchange_id>/state/open-registration", methods=["POST"])
+@admin_required
+def open_registration(exchange_id):
+    """Open registration for an exchange.
+
+    Changes exchange state from draft to registration_open.
+
+    Args:
+        exchange_id: ID of the exchange.
+
+    Returns:
+        Redirect to exchange detail page.
+    """
+    exchange = db.session.query(Exchange).get_or_404(exchange_id)
+
+    # Validate current state
+    if exchange.state != Exchange.STATE_DRAFT:
+        flash(
+            "Registration can only be opened from Draft state.",
+            "error",
+        )
+        return redirect(url_for("admin.view_exchange", exchange_id=exchange.id))
+
+    # Update state
+    exchange.state = Exchange.STATE_REGISTRATION_OPEN
+    db.session.commit()
+
+    flash("Registration is now open!", "success")
+    return redirect(url_for("admin.view_exchange", exchange_id=exchange.id))