feat: implement location events & error handling (Step 10.2)

- Add LocationService with create/rename/archive methods
- Add LocationProjection for event handling
- Add admin-only location management routes at /locations
- Add error response helpers (error_response, error_toast, success_toast)
- Add toast handler JS to base template for HX-Trigger notifications
- Update seeds.py to emit LocationCreated events per spec §23
- Archived locations block animal moves with 422 error

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_web_responses.py

@@ -0,0 +1,109 @@
+# ABOUTME: Tests for web response helper functions.
+# ABOUTME: Tests error_response, error_toast, and success_toast helpers.
+
+import json
+
+from starlette.responses import HTMLResponse
+
+from animaltrack.web.responses import error_response, error_toast, success_toast
+
+
+class TestErrorResponse:
+    """Tests for error_response()."""
+
+    def test_returns_html_response(self):
+        """error_response returns an HTMLResponse."""
+        response = error_response("<div>Error</div>")
+        assert isinstance(response, HTMLResponse)
+
+    def test_default_status_422(self):
+        """error_response defaults to 422 status code."""
+        response = error_response("<div>Validation error</div>")
+        assert response.status_code == 422
+
+    def test_custom_status_code(self):
+        """error_response accepts custom status code."""
+        response = error_response("<div>Conflict</div>", status_code=409)
+        assert response.status_code == 409
+
+    def test_403_status_code(self):
+        """error_response works with 403 status code."""
+        response = error_response("<div>Forbidden</div>", status_code=403)
+        assert response.status_code == 403
+
+    def test_401_status_code(self):
+        """error_response works with 401 status code."""
+        response = error_response("<div>Unauthorized</div>", status_code=401)
+        assert response.status_code == 401
+
+    def test_content_in_body(self):
+        """error_response includes content in body."""
+        response = error_response("<p>Error message</p>")
+        assert b"<p>Error message</p>" in response.body
+
+
+class TestErrorToast:
+    """Tests for error_toast()."""
+
+    def test_returns_html_response(self):
+        """error_toast returns an HTMLResponse."""
+        response = error_toast("Something went wrong")
+        assert isinstance(response, HTMLResponse)
+
+    def test_default_status_422(self):
+        """error_toast defaults to 422 status code."""
+        response = error_toast("Validation failed")
+        assert response.status_code == 422
+
+    def test_custom_status_code(self):
+        """error_toast accepts custom status code."""
+        response = error_toast("Not allowed", status_code=403)
+        assert response.status_code == 403
+
+    def test_has_hx_trigger_header(self):
+        """error_toast sets HX-Trigger header."""
+        response = error_toast("Error message")
+        assert "HX-Trigger" in response.headers
+
+    def test_toast_message_in_header(self):
+        """error_toast includes message in HX-Trigger header."""
+        response = error_toast("Error message")
+        trigger = json.loads(response.headers["HX-Trigger"])
+        assert trigger["showToast"]["message"] == "Error message"
+
+    def test_toast_type_error(self):
+        """error_toast sets toast type to error."""
+        response = error_toast("Error message")
+        trigger = json.loads(response.headers["HX-Trigger"])
+        assert trigger["showToast"]["type"] == "error"
+
+
+class TestSuccessToast:
+    """Tests for success_toast()."""
+
+    def test_returns_dict(self):
+        """success_toast returns a dict (for HX-Trigger header)."""
+        result = success_toast("Action completed")
+        assert isinstance(result, dict)
+
+    def test_has_show_toast_key(self):
+        """success_toast dict has showToast key."""
+        result = success_toast("Action completed")
+        assert "showToast" in result
+
+    def test_message_in_toast(self):
+        """success_toast includes message."""
+        result = success_toast("Created successfully")
+        assert result["showToast"]["message"] == "Created successfully"
+
+    def test_toast_type_success(self):
+        """success_toast sets toast type to success."""
+        result = success_toast("Done")
+        assert result["showToast"]["type"] == "success"
+
+    def test_can_json_dumps(self):
+        """success_toast result can be JSON serialized."""
+        result = success_toast("Test message")
+        serialized = json.dumps(result)
+        assert '"showToast"' in serialized
+        assert '"Test message"' in serialized